Use rules to block specific field entries?

I'm fairly new to rules, but I've used it successfully to add a ton of great features to my site -- great module!

Now I'm trying to figure out if there's a way I can use Rules as part of a reservation system I created. Basically, I'm trying to create a rule that will disallow users from reserving equipment on a date if someone else has already reserved the equipment for that date. Is this possible with rules? Any ideas for ways I might test it out?

Any help is much appreciated. Thanks!