You can't prevent them with this tactics, because they will always add just one more letter or change number and add identical image with description to look almost identical to original username.
Telegram is notorious for this, but Discord is even worse, and there you can create totally identical username with only ID is different, but you need to change settings to enable visible ID.
Yeah, I understand that, and I did allude to that being the case. However, it's
probably a decent idea to at least get the matching usernames on as many platforms as possible. I mean it doesn't exactly hurt, if you have a unique username just register them you don't have to use them. Although, yeah you'll never stop others impersonating users, and unfortunately with the different characters some platforms allow, some can pretty much look undistinguishable.