Currently, there is no way to prevent unwanted guests from spamming clan chats with F2P ALTs unless you disable guesting. A total level requirement would solve this problem.
In order to prevent harassment like this https://i.imgur.com/UkPLkWy.png , this https://i.imgur.com/nUC6RRc.png and this https://i.imgur.com/fi9EY3X.png to Clan Chats without permanently disabling guesting, Jagex should give Clan Leaders the ability to add a total level requirement to join Clan Chats as a Guest.
Mod Shauny has come up with a brilliant PIN idea, which is currently awaiting QA, however I do not believe that a PIN alone will solve the problem because PINs can easily be leaked and a PIN will also ban all players from the clan chat who do not know the PIN.
We want for guests who we have never met before to be able to join clan chats as a guest, which a total level requirement would allow, but a clan PIN would not allow. Therefore, the best solution to counter in-game harassment/cyber bullying via guesting in clan chats is to give clan ranks the ability to set a total level requirement for guesting in clan chats and joining clans.
I believe that this is a very necessary quality of life clan improvement and should be made a priority.
Without delving into to much (overpromising, etc) I have a plan for Clan QoL update 2 which will try to do exactly what you’re asking for
The update itself is awaiting QA but it’s quite a large one due to the multitude of new features on it
Hang tight we’re aware of the issue and hopefully it’ll release to literally help with this issue!