Yeah these features would be very handy. It is very annoying when people in multiple converstaions constantly invite you back in after leaving and u have to block them one by one

. I wouldn't call it /chatblockusers tho, i'd name the tag maybe: /blockall or /blockconvo.
Your second idea is also good, maybe not as useful for some people

, i'd call it maybe: /blockmulti or sumthing with multi in it so users know that they are
actually blocking multi convo's, not everything

that would ruin the point of signing in

lol