If I was admin I would update alliance strength to work similar to nation strength. This would act as an overall level of strength that would limit the target alliances an AA can declare upon based on this number by either making it impossible for the AA to declare war and/or put into effect, either immediately or over time, attrition modifiers that would make it increasingly harder for larger AAs specifically in soldier/tank battles. This wouldn't be a big deal since the larger nations of big AAs that declare upon smaller AAs tend to just sit back and act as banks for their underlings anyway- the warchest of these "bank" nations would drain much faster as a result of the modifier as more war aid would be required.
Sorry, guys, but I agree with Johnny Apoc. Hate me for it all you want.