Are bulletin boards intrinsically not xhtml compliant?

[Robink]

Even though all my vb3 templates are xhtml compliant, when I validate threads, the validator will inevitably give me a "Not compliant" result.

And the reason for this is, that users who post on the forums tend to use non-SGML characters like the euro sign(?), or Quotes like this: ?, ?, ?.

And since they can use vbCode, they often write code like:

[b][i][u]hello[/u][/i][u][/u][/b][u][/u]

Since a bulletin board must have some part of its pages entered by users, and you cant expect all the users to write correct well nested tags.. my pages are always non xhtml compliant.

Is there a solution to this?
Or due to the very nature of a bulletin board (contribution of user-written content), can it never be xhtml compliant?