Try the flag a concern thread if u need assistance of a Jmod for it. It is also adviced to not change ranksystem rules immidiatly, but let them know a while beforehand and give them the option to obtain what is needed extra then.
Ceecs - leader of Ceecs Clan
Rank in Star Find, Harmed Ore, Beach FC | Ex-Rank in Fast SC
Ceecs Clan: DUTCH ONLY RS3 clan & Winner of the Iron Skilled Cup 2015!
this same thing happened to me . while I was away at a family funeral some1 made name lincolnlady 1 letter different then mine capitol i instead of L. and scammed some1 in my clan for 100m. now I feel I don't want to even be in cc or on rs. this has really sickened me
All you can do is ignore and ban, unfortunately Jagex are really really poor when it comes to dealing with people who are offensive on the game so this kind of behaviour is allowed to breed quite freely.
Consider muting guests rather than disabling it completely.
This way when someone you feel is a genuine guest is guesting, you'd simply (ask to) PM them and explain the reasoning behind it, and kick those you recognise as trolls. Muted guests would be able to get a feel of the clan and chat environment at least.
Ultimately though I'd exhaust all other options before giving these trolls the self-validation and acknowledgement as trolls by handicapping myself with the clan settings.