Example of a PlayerCanHearPlayersVoice hook:
local function GagPlayer( listener, talker )
if not talker:IsAdmin() then
return false
end
end
hook.Add( "PlayerCanHearPlayersVoice", "gag_player", GagPlayer )
This will make only players who are admins able to talk.
Beware, using too many of these hooks can cause server lag as they are very intensive.