Blocking a service obviously is not allowed and barely present in competitive volleyball. but it is one of the possible errors. how will the main referee show that (as per, brushing finger tips for block-out). What the rule book saying about that.
To quote the Official Rules, page 35:
14.5 BLOCKING THE SERVICE 12, D11 (12)
To block an opponent's service is forbidden.
"D11 (12)" is diagram 11 "Referee's official hand signals", signal 12 (page 74):
Blocking fault or screening
Raise both arms vertically, palms forward
i.e. the standard signal for an illegal block.