Since I haven't tried LOMAC online, I can't really comment on how big a problem AMRAAM-spamming is, nor how uber the F-15 is.
However, if the game or match is unbalanced, then that's a design flaw in the setup and that should be addressed by modifying the setup of the match (or scenario.) The answer (this applies to wargames as well as simulations) to this is to change the setup to be more balanced rather than to change the parameters of the platforms and weapons. For example, if the F-15 + AMRAAM really is (and I'm not saying it is or not) at a significant advantage over the Su-27 in real life, then that should be reflected in the game. So to make it balanced, rather than trying to fiddle with the AIM-120 parameters to make it less useful, rather the Russian side should probably get more Su-27's or an AWACS or some other advantage to make it balanced. If the game developer started messing with parameters of missiles to make the game more "balanced" rather than realistic, then the game would cease to be a simulation of anything. Heck, in that case, you might as well just replace the skin of the F-15 3-d model with the model of the Su-27 and pretend it's the Su-27.
That said, if the game allows unrealistic tactics to be used (missile-spamming, for example) successfully and without any penalty, then that's a design problem with the game that should be addressed if possible. A good sim design would reward good, realistic tactics with success and punish unrealistic or bad tactics.
