Multiple website with Buddypress

Hi,

Problem – I do not want my sub site users of on buddypress site to access another sub site. But access the sub sites which I wanted.

How I config now –
1.Activated Multisite as sub domain based.
2.site1.mainsite.com – Installed Buddypress in this sub site
3.site2.mainsite.com – Installed Buddypress in this sub site also

Now each sub site will be mapped with an new domain like, site1.mainsite.com = site1.com

What is happening – The users of site1.mainsite.com have access to site2.mainsite.com which is due to the nature of multisite I think. Is there a way to block this?

For eg- user1 from site1.mainsite.com cannot access other sub sites but can access specified sub sites like general.maninsite.com where users of all sub sites have access to this site.

Any solution?