if this is just a few clients and you can identify them, you can isolate them by creating a new vserver as the exact copy of the current except you will add the command 'client x.x.x.x' and a new serverfarm that will also be a duplicate of the existing one but with the command 'maxconns' to limit the number of connections for these clients.
This is to my knowledge the closest we can do to what you need.
Regards,
Gilles.