In this use case, that is probably a bad idea in my experience. Why? Well I hope this makes any sense:
You need to allow the optimization algorithms to break it now and then to tunnel through a bad search space into another good search space.
If it doesn't, don't worry.

Hmm, I think I understood it. Allowing infeasible solutions may help to scape from local minima in the space of feasible solutions for example.

rule "Avoid conflicting activities"

when 

Assignment($room1 : room, $act1: activity, $id : activity.id)

Assignment(room== $room1, room != null, $act2 : activity, activity.id > $id)

Conflict(act1 == $act1, act2 == $act2)

I would put Conflict first. But try it this way too and let me know which works better ;) I don't know.
Stated differently: Instead of checking every 2 simultaneous assignments if they are a conflict,
I would check if every 2 conflict assignments are simultaneous (like in examinationScoreRules.drl).

Ok! I will perform some stress tests to verify which one works better.