I also want to know how do i create a plugin that would suspend users from adding or messaging people on my network similar to what face book does. :)
profile manmager, users can block other users on elgg if you want some custom plugins go to the professional services group and get what ever you need : ) please release any plugins you get made to elgg!
For your first question regarding a "premium plugin" I can't help you. There are a few available (mostly commercial on other sites) but I don't know what exactly they can do. You might want to do a Google search to find the premium account on other sites and ask their developers about them.
For blocking users:
http://community.elgg.org/plugins/1164584/1.0.7/block-user (not tested myself; might not work anymore on the latest releases of Elgg)
http://community.elgg.org/plugins/385010/1.8f/messages-with-privacy-option-more (not tested myself; might not work anymore on the latest releases of Elgg)
Then there's a plugin for Elgg 1.7 (which means it most likely will not work anymore on Elgg 1.8 and would need to be ugraded if not): http://community.elgg.org/plugins/475802/1.0/messages-controls
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