I just downloaded your client to do some testing, and oh god that client is really disturbing.
Apparently your can't disable the join/quit messages in the main log (on the left of the screen).
I managed to delete the join/quit messages and the system messages in the sub-log (on the right):
- Uncheck "Also output system message to sub-log" in the osu! server settings (important)
- And for every channel, go to "Edit Channel Setting" then uncheck or not "Output to sub-log always" depending on what you want to see in the sub-log (and, if you want to, uncheck "Output text to sub-log" but you won't be able to see the new user posts of this channel in the sub-log)
So with these parameters:
- When I open #osu on the left: I see the join/quit messages + user posts of #osu on the left, and I see new user posts in the other open channels in the sub-log (i.e. only user posts, not system and join/quit messages)
- When I open a different channel on the left (e.g. #russian): I see the join/quit messages + user posts of #russian on the left, and new user posts in #osu and other open channels appear on the right
I hope that could help you.