There are a few things you can do here. The easiest would be to set the following site setting to -1:
The only downside is that you won’t get reminded if there are pending users that have been waiting for the specified number of hours. As long as someone is usually around that can approve new users, it shouldn’t be a problem.
The alternative would be to either delete or approve the users from the command line. Let me know if one of these approaches is preferred. I can work up the commands for you on either.