If that is not an option, then you're out of luck. The last thing I heard about this was in august 2015, where this tweet was made by Slack on Twitter:
@SlackHQ: can we hide join/leave messages through global or channel
settings?
@willimac not at the moment but we're glad to send your
request over to our team!
Open a direct message and STAR it to keep it showing on the sidebar. This is very useful because it does remove any duplicates that would show up in the direct messages part of the sidebar, so you still have plenty of space.
I don't think that is possible. But you could make use of a clipboard manager app, and then individually copy all the messages one by one. The clipboard manager app should be able to store all the messages separately, and then later you can paste them where you want.
Maccy or CopyClip are a couple of examples of clipboard managers for Mac.
Best Answer
UPDATE: It seems it's now possible
Not as it is at the moment, no, unfortunately.
One thing you can do, is to mute the channel.
If that is not an option, then you're out of luck. The last thing I heard about this was in august 2015, where this tweet was made by Slack on Twitter:
I myself have been using this userstyle as I'm only using Slack for WordPress: https://userstyles.org/styles/106647/wordpress-slack-hide-join-leave. I don't know if this is something you will be able to modify or user for other use cases.
WordPress also links to this at their official page about Slack, so I'm guessing that there isn't an official solution out there yet :/