Blacklisting feature for Space Owners

I propose creating at least a basic blacklisting feature available for Space Owners to limit hostile, offensive and annoying users.

It could be as simple as the one already used for adding or removing friends and would prevent unwanted users from reentering the Owner's Space(s) (until revoked).

In the future a more advanced system could also be developed: kicking, banning, setting exact time for bans etc.