I wouldn’t do it that way. If you’re really concerned, take backups of the database at intervals that yo uthink are relevant and save them on your own PC (or somewhere else that you have access to and the devleoper doesn’t). The same can, and should, be done with the websites files.
At the end of it all if you really want to lock the devleoepr out, then you can remove their admin account from WordPress, then change the FTP/control panel/etc passwords. That’s going to be enough to stop them from gaining access normally.
Then again, if you really want them to have no access, you can get them to set up the theme, and email that to you and you can install the theme and populate and set up the site. That way they won’t have any access at all to it.
If you are really that worried you might want to check that the developer hasn’t built in any back doors to the site. It’s an extremely remote possibility, and would only b edone by the most un-ethical of developers, but it all depends on just how paranoid you feel at the time.
In reality though, you shouldn’t have to do that. There’s not many developers out there that will do anything bad at all. It only just some of the worse stories that you hear sometimes. If you’re really going to be that worried about your devleoper, spend the extra money and hire osmeone that doesn’t make you nervous. It will be well worth it in te long run!