In this question, the user admits to be using a newly create account because
SO has somehow locked me out of my account and I apparently can't post images with this new one
What's the best channel for notifying the right people?
Flag the post and leave a note for the moderators.
In general, though, I don't think this is a big problem. Using a new account to get around an actual timed suspension aside...
Having multiple accounts isn't always a bad thing. Sure, using one to get around a question block on another isn't exactly encouraged, but if the user has learned from their mistakes... there's no need to penalize them. And if they haven't, they'll get question-blocked again in due time.
These things tend to have a way of working themselves out.
Flag the post for moderator attention. They will check that and take appropiate actions if necesarry.