Without seeing the HTML code it's hard to suggest a search a proper regex.
Switch to code view mode and locate the number. If it looks like this:
select the regular expression mode (Regex) and use the following expression:
and replace with nothing.
For upper case roman numerals use:
Note that you can't use this expression with replace all, because it might also match certain upper case words.
For more examples, see the
Sigil Regex thread.