I believe these members signed up via Tapatalk free version. The first minute they sign up using Tapatalk (specifically free version), "Using Tapatalk" will be injected into members' status update and signature. Apparently there's a big registration loophole. People can sign up using TapatalkID and via Sign In option (even they don't have account on your forum).
The only solution now is to disable/remove completely Tapatalk plugin on your forum since the "in-app registration" adminCP option has been removed and it won't help even if you alter the config.txt file to disable registrations. Tapatalk developers themselves said they are trying to improve the in-app registration. We'll see. Many forum owners have already removed Tapatalk plugin from their forums. I am near to remove Tapatalk plugin.
In short, this is non-xenForo related issue. You might have posted in wrong section.