Not overbooking is one of the hardest things to do. You often make a lot more money on your last customers than on your earlier ones so the temptation is very high.
One of the first things I do on a trip is figure out the ideal group size (based on overall guest experience, site/facility/vendor capacities, and of course financials) and make a hard limit of how far over I can go. That limit is normally just a few people more, but for a larger program it can obviously scale a bit. But you must know your limits and what your customer is expecting and never cross those lines. I turn people away all the time; even if I sometimes have the capacity it's not fair to the existing group to have an experience different than what they signed up for.