Hmmm, you'd have to do a join query to get the username from the username table but in order to get a list of those with links in thier signature this should work in admincp>maintainance>execute sql query
PHP Code:
select distinct f.username
from vb_user f
inner join vb_sigparsed u
on u.userid = f.userid
where u.userid= f.userid AND signatureparsed like"%<a href%" OR signatureparsed like"%http://%" OR signatureparsed like "%www.%"
dont forget to change
vb_ for your table prefix or remove it altogether if you dont use one.