Talk to your HR department. This is the MOST
important thing to do. Explain the situation,
and that your preference is separation as
fast as possible.

They need to be involved as soon as they can.
There are a lot of legal and political issues
to be dealt with, and they are trained / have
experience in that area

Then take stock of why you thought he interviewed
well. Because you WILL make this mistake again
unless you identify the hole in your process.

Not the mistake of hiring a slacker who doesn't
call in sick when he should, or an idiot who
wants to sell parts on E-Bay. This type of stuff
is probably impossible to check, unless you believe
in the personality tests.

But the mistake of hiring a CNA who thinks Netware
is part of Win95? C'mon?! This is your fault if
you tech proofed him.

I find tech interviews go far better (for me) if
someone else is asking the list of questions and
engaging in conversation. I observe and am able
to pickup the odd things and contradictions much
better that way, and ask the occasional deep dive
questions.

Some people have gotten annoyed with me, told me I
was too much of a hard ass. Far better to not let
the semi-competants through the door, than have
to fire them after they've screwed up and are about
to take you down with them.

The MOST important thing a manager does is hire
qualified people. Without that, you've got nothing to
manage!

You might want to get your boss involved for your career
training in this area. Because a couple of more people
like this, without any intervening winners, will get
YOU fired.