Implement "gang point counter"

1) Each time someone participates in a gang, he gains +1gp ( gang point )

2) Each time someone is ganged, he loses 1gp. ( until zero )

3) A character with zero gp counter can't be attacked by more than 2 people at once. ( Third one can't initiate a combat ). A character with positive gp counter can be ganged freely.
Special bonus: Those who keep their gp counter at zero for a while ( 10 last fights, for example, or 80% of time ) can be attacked only by a solo person.

4) If a person attacks a group, groupmates can assist each other without gp restrictions. ( However they gain their gang points if they kick the offender's ass together ).

This is just a general idea, it has flaws. Feel free to polish it.
You can also ask Valguarnera to provide you with roleplaying bla-bla justification.

P.S. This is a half-joke, however, it makes sense.
You will get much less people who claim that they are always fighting solo while everybody else gangs them.