Honestly just requiring a FC to maintain a 4 person membership. If you have more then 1 character from the same service account join the FC; if you leave with 1 character you automatically leave with all of them.

Worst case scenario? You have someone in a "real" FC that gets inconvenienced because they want to pull themselves and their alts from the FC so they lose their rooms. This would mean a single service account could only maintain 2 Shell Fcs; so 4x less. Sure some folks are going to spend money for additional service accounts; but honestly your not going to make it impossible for folks to throw money at SE for stuff.

I'd also take it a step further and require new FC members be required to earn X amount of company credits with the FC before FC ownership can be transferred to them. This would be in addition to the 30 day transfer min requirement.