Can't buy into more than one usergroup?

[Aryil]

Sorry if this has already been posted, but I looked and can't see it.

I'm using the buy into usergroup module, so that my users can buy VIP. The thing is, there are four different VIP colors, and I want it so that my users can only buy into one color at a time, to prevent their 'subscription' to a specific color of VIP being lost.

I'm not sure if that makes sense, so I'll try putting in differently.

Say the VIP groups are A, B, C and D.
I have user Bob, and he buys into group A. Using the 'Enforce buy limits within this time' option, Bob can't buy into that usergroup again for a month, because that's how long his membership lasts.
But he can buy into groups B, C or D, thus losing his membership of group A.

I want to prevent users from buying into a usergroup once they have brought membership for one of the groups, until their membership runs out.

If anyone could tell me how to do it, I would really appreciate it!
Thanks.