It's acceptable to block someone online.
Greg's kid Gus likes to hang out on online chatrooms and talk to strangers, and Greg notices Gus has been trying to meet one of them.
Greg wants to address the situation.
Greg blocks the stranger from chatting with Gus using a chatroom blocking feature.
Greg tells Gus he can't chat online with people anymore.
Gus notices the stranger doesn't seem to be around any longer, and gives up trying to meet him.
Gus secretly sneaks on to the PC when Greg's not around, and sets up a meeting with the stranger anyway.