Personally I wouldn't end this behaviour. That would be a waste of a perfectly good opportunity for a GM to have the PCs wriggling on a hook, smug in the knowledge that they put themselves there.
Step One. Don't stop the activity. If anything allow the PCs to get some pretty significant material benefits from it.
Step Two. Expand. Ensure that more and more bad guys are on the PCs books. Some of these guys are actually pretty high ranking in various increasingly sinister and malevolent cults. All are able to offer the PCs further high ranking cult members and material rewards to save their own sorry skins.
Before long the PCs are turning a nice profit and have mapped out a good section of the cult activity in the area.
Step Three. Things start unravelling. Adeptus Arbites, Enforcers and independent bounty hunters start investigating some irregularities. For example the manufactorum that was set up was only approved because unbeknownst to the PCs the noble murdered an administratum official who was obstructing the planning procedures. Now the noble wants help in delaying the investigation.
At the same time another cult leader or cold trader is having problems with an even worse threat, say a khorne mutant warband and asks that the PCs devote resources to destroying it.
No doubt the PCs think these actions are ok to protect their position, after all it is only pulling rank a little on some Enforcers and killing mutants. But the wider picture is they are becoming the patrons and leaders of the cults they are running.
Step Four. Ends Justify the Means.
The pressures of keeping all the blackmails, informants and off the book deals on the go begins to grow. The cult leaders contacts etc start making increasingly bold actions to advance their own position on the basis that there is a cell of acolytes protecting them.
For example one of the cult members informs the PCs that there is a particularly mad sorcerer in charge of his group. If he could be deposed of it could be a great victory for the Imperium. Of course it would also give said cult member a chance to advance. The PCs again cross a line, now they are justifying influencing cult politics to advance their own contact.
Step Five. Increase the Pressure. Rising undealt with cult activity has come to the attention of their Inquisitor. He wants results and asks the PCs to team up with another acolyte group (preferably this can include new PCs) to deal with the issue. How do the PCs decide which bridges to burn? How do they deal with these loyal untainted Acolytes. Whatever they do they’ll lose influence and potentially expose themselves.
Step Six: Corruption. A cult leader or xenos smuggler aware of the PCs predicament offers them resources to help them keep ahead of their master. Perhaps a sorcerous technique for scrying or heretical technological device that can alter data records.
The PCs are now running their own cult(s) and are directly opposed to the Inquisition. And they’ll be wondering where it all went wrong.
And that my friend is Dark Heresy.
However that said a few courses of action spring to mind to end the matter.
1) The guys being blackmailed are in fact Inquisitorial Acolytes belonging to an enemy Inquisitor and the whole thing is actually a sting operation. The tables are now turned and it is the PCs are the ones being forced to betray their masters
2) The nobles who are members of cults might call on their cults for support. Facing the wrath of the Inquisition might be just the tipping point needed to turn a hesitant cult member into a full blown initiate who is willing to sell his soul for protection. Perhaps a few weeks later the PCs start getting plagued by a daemonic assailant (maybe even a daemon host) trying to uphold its side of the bargain.
3) Depending on how heinous the crime the noble calls in favours to get him off the hook. Sure if the noble is full on calling up daemons only other cult members will help him but if say he is taking a share of a fairly minor xenos smuggling ring or even a member of a cult which is dodgy but has only been revealed to be outright heretical by the PCs there might be wriggle room.
E.g Noble goes to his brother who is of course the local Bishop and 'confesses' his sins. Bishop is willing to grant absolution on completion of 'very arduous' penance carried out under supervision of course and or the Bishop agrees to declare that the noble was working under his orders to infiltrate a potentially heretical cell in the city and the whole thing was a misunderstanding.
Basically the noble might go public and be able to survive by the skin of his teeth because of his connections.
The Acolytes on the other hand likely only have the protection of their Inquisitor which is now undermined because of their activities. Even if the Inquisitor allows for their transgressions they have made a powerful enemy in the Noble family.
Finally as has been said before I wouldn't necessarily think that blackmailing such low level cult members is such a problem. It depends whether the Acolytes are furthering the aims of the Inquisition or simply making a profit.
Edited by Visitor Q, 14 March 2014 - 04:20 AM.