If you click the ⋮button on the right of a user's profile, you`ll see a 'Block this user' link. Clicking this link takes you to a confirmation page, describing exactly what the block means.
The blocked user will no longer be able to do any of the following:
join groups you own;
send you messages from your profile (if you have that option turned on in your settings);
comment on your group discussions.
However, blocking does not retroactively affect most of these. Any of your scores previously favorited by a blocked user will still be favorited, and their comments on your scores will not be automatically deleted. The blocked user will be retroactively removed from groups you own, but their scores and discussions or comments will not be automatically deleted.
Note that profiles are public, and blocked users can still view and download your scores (except those that are private).
If you subsequently want to unblock the user, there will be an 'Unblock this user' link in the same place on their profile that the original 'Block' link was.
Keywords: block, blocking, mute.