Valid XHTML - Problem with usernames

I don't know if this is the correct forum to post (sorry if not), because isn't strictly related to VBSeo. I'm trying to validate my forum as XHTML 1.0 Transitional, and I'm having some troubles with some usernames with strange characters like: ★€k€p★

w3.org says:
"non SGML character number 128."

Is there some way to change that characters in the code without changing the username? In the HTML appears like:



Edit: Mmm, that's strange... With that same username, sometimes validates, some others not... It could appear as:



Or

<- And that's VALID XHTML 1.0 ...

It seems that sometimes the " " character appears, but no everytime...

In his Profile Page appears correct (XHTML Valid), but in the list of OnLine users does not (Not Valid XHTML)



Thanks