After checking in a 1.2.0-beta5 (gnusocial.cc), it seems that silencing another user as a regular user is not possible; only admin can do it, and globally (silencing for all users).
This seems to be a feature only present on Qvitter. Right now, with 2.0-dev, only "Block user" is available for a regular account. I don't really know if it is a bug, or is not implemented. Anyway, ModPlus plugin allows to see this clearly.