curl -sS -X POST 'https://matrix.example.com/_matrix/client/v3/rooms/!abc123:example.com/ban' -H "Authorization: Bearer $ACCESS_TOKEN" -d '{"user_id":"@bob:example.com","reason":"abuse"}'

Kicks the user and blocks them from rejoining until unbanned, which makes it the standard tool against abusers. The reason is recorded with the ban event. Unban by setting their membership to leave via the m.room.member state endpoint. Room-level bans only affect this room; server-level controls live in the Admin API.