You're mistaking 2 different situations, assuming you're talking about something that prevents targeting, like Forbidden Dress. If you chain something that prevents targeting to a card that has already selected its target, then that card won't prevent the targeting, since it has already happened. Dracossack targets as part of its activation, and if you chain Safe Zone, since it resolves first, it will prevent the targeted monster from being destroyed.
Example. Monster attacks - Chain Link 1: Dimensional Prison (it targets at this point)
Chain Link 2: Forbidden Dress (targets the attacking monster at this point)
Chain Link 2: The target cannot be targeted or destroyed until end of turn.
Chain Link 1: It has already been targeted by Dimensional Prison, so it is still banished.
The common misconception here is that some people think that Forbidden Dress will prevent Dimensional Prison from working since it prevents targeting.
Edited by MadRest, 22 August 2013 - 11:18 AM.