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.

[ZiNgA BuRgA]

If the only difference is the colour of the usernames, you could use a separate "Custom Username Formatting" item.
Or you can place instructions telling users not to buy into more than one usergroup, unless they want to waste their money.
Third option is to use secondary usergroups, but I'm not sure if that gets your desired result.