At the moment, there's no single query or user-interface feature to do this. There is, though, a function that does what you want: thankyoulike_delete_user() in inc/plugins/thankyoulike.php. Ordinarily, it is called when a user is deleted, but you could arrange for it to be called for an existing user to remove that user's likes if you're handy with code.
Edit: you would be best off recounting thanks/likes via the ACP afterwards though because the function assumes that the user is being deleted, so it doesn't adjust his/her thanks/likes counts in the user table.
Edit: you would be best off recounting thanks/likes via the ACP afterwards though because the function assumes that the user is being deleted, so it doesn't adjust his/her thanks/likes counts in the user table.