Although this is always been a significant problem in Teeworlds, (I guess you aren't a "real" clan these days without having at least 3 clan name fakers) I think it's difficult to battle it within the current setting.
This case is especially difficult, because with them being on protected server they are not attempting to defame the first clan to the public. They seem to do this just because they prefer the name, and considering it's no longer taken this is a difficult case to prove your rights.
Therefor I think the main problem is that it's questionable how much rights you can have on a specific clanname, especially when it's no longer in use. Teeworlds has never officially supported clans. Strictly spoken people just started writing certain letter combo's in front of their name to show they formed an alliance. Certain clans got very professional, contributed a lot to the community and did develop a very good reputation. But it still remains a letter combination in front of a name. There's no "copyright" whatsoever on it.
To say you own a clanname, even when you no longer use it, goes a bit too far in my opinion. Teeworlds allows you to take any name you want, and to name your server anything you want. There's no serious clan scene like many other games do have, it are just groups of friends, playing together because they feel connected in on way or another. Peoples (clan)names can change dozens of times, but couldn't possibly hold on to the ownership of all these names. To say that people shouldn't impersonate your current clanname is perfectly just, things would turn into chaos if this wouldn't be a fact. But once you drop a certain "letter combo" it's free to the public.
As long as these people don't actively try to dishonor the reputation the previous clan had, there isn't much to complain about. Sure, they might not do as much as the old clan did, but the community knows that you moved on with your new name, and will discover by themselves that the clan that goes by the old name might not be exactly the same as it's predecessor. Your reputation will stay the same, because it moved together with you to the new clanname.
My advice is to friendly discuss this matter with the persons in question in game. If they are willing to conform to your suggestions then the problem might get solved. If they disagree though you better restore the reputation of the old clan by proving this with your new clan. You decided to change your clanname, just show the community that the people behind it didn't. Sure your old name might not live up to the reputation you once had, but then again, how much reputation could non-existent clan still have in a few months? At least in this way your name will stay alive and possible contribute something good in the future.
