Pro Sites another wierd question for you

I have 3 subdomains that are on the network.
However I dont want users who register on those sites rather than the main domain to have anymore than subscriber level, basically they are a job site a directory site and a classifieds site, so i dont want the user to be able to post via the backend dashboard.

I could have done the installs as standalones but wanted the users to be able to move within the network without the need for multi logins.

All three sites have full front ends and have no need for back end use by users. I could hide the pages admin bars at the top but that wouldnt stop the my sites access.

Any ideas? This is stopping me going live at the moment.

Thanks

Tom