I have a user stuck on the server somewhere not in a channel,
if I do a user list I get this:

-[ OperServ ]- avejou Lisa@2F78A622.95B10E7B.83D92844.IP

so she cannot log in with her normal username. How do I get rid of her please?

Consult the documentation for your ircd or services, probably a /kill command.

depending on what IRCd server you're using

or perform a /whowas avejou which will give them their details

if it's unrealircd, as an IRC operator

/gline - which will give you all the list of users banned

find avejou Lisa with their real credentials and not hidden

then /gline -host@mask

if you already done this, well done.