[request] Can´t answer in Thread after "X" Posts

[Jumper]

Hi,

i need a mod or plugin that some usergroups can´t answer in thread after x posts.

example:

one thread has 4 replies (complete 5 posts). if now one user from usergroup 11 will answer, he must get a error-message cause the limit of 5 posts is reached.

i need it for usergroups 11 & 12 and only in forums with fid 6 & 7.

I would be happy if someone can help.