When can the warrior still ditch his item?
1) before the thief rolls the die
2) After the thief rolls the die
3) After the thief lands on the warrior's space
4) after the thief declairs he is going to encounter the warrior
5) after the thief declairs he is going to steal from the warrior
Option 3 is the best one. But you need to be quick.
As soon as the Thief says, that he is gonna steal, then it´s fair, that you may not ditch the item anymore.
Besides, in the scenario described in the above posts, options 2 and 3 would simply allow the thief to encounter the space with the ditched item.
This is not so easily solved out as it seems. I think option 1 will look illogical to most players, because it has no sense to ditch an Object to prevent a stealing that maybe will not occur; nevertheless, it's the only solution that does not create problems in handling it.
zealot points out that 2 and 3 would allow the Thief to encounter the space instead ad simply pickup the Object in most cases. Velhart suggests that option 5 could lead to potential problems (as well as option 4, IMHO). Problems generated by the Warrior ditching the desired Object or, potentially, all his Objects leaving the Thief with no Objects to steal (5) or forcing the Thief to declare an attack (4).
I would say the only legal way to ditch Objects to prevent the Thief from stealing is 1 (2 and 3 are also legal, but pointless in most cases).I'm sure nobody will go for it, so don't ditch Objects at all. Alchemize them if you can before the Thief declares to encounter your Character.
Same goes for Spells. Who says "I cast a Spell" first solves it first, barring the exceptions of Counterspell and Reflection Spell. You cannot play a Spell before an already casted Spell takes effect, and you can't do anything else except resolving the Spell. So the only available option is 1), if you consider it a viable choice (maybe for the Prophetess, who always knows which Spells are in each player's hands).