If a user is suspended from Stack Exchange, couldn't they just create a new account with a different email address and then just merge the two accounts after their suspension? This would completely undermine the point of a suspension, so I feel like there is something set in place against that. If so, what is it?
They can try. They will fail.
From the FAQ:
If the account holder creates a new account to bypass the suspension, that new account will also receive the same suspension period as the previous one, even if the old account was deleted.
Suspension also takes into account IP address and other non-public means to catch those who try to trick the system.