Either a dealer or a well-equipped locksmith should be able to reset the security system and reprogram it for any fobs you still have. The fobs operate at a fixed frequency and the car is "trained" to accept their signals. My local locksmith charges $30 for doing this.
My wife lost her keys and with them the keyless entry fob. Is there any way to deprogram the van from accepting that fob?
If you have one of the trims that uses the "smart" keys with the microchip in them (like mine does), be prepared for some sticker-shock: the key blanks go for about $30 and they also need to be trained.
Replacement fobs can be found on eBay for lots less than a dealer will charge for a new one, and there are plenty of instructions here and around the Internet on how to train the car for the new fob.
Worst case is having to re-key the vehicle. ($$$$$$) Chrysler keys are fairly distinctive and a determined person could go through the apartment complex trying the keys in all the suitable vehicles. This does presume that your wife is confident that she lost the keys somewhere in your complex, and that they've fallen into disreputable hands though..
'01 Town & Country Limited, 150K miles & still rolling
'97 Saturn SL2