Firstly this would depend on what is the e-mail service that is doing that, it may automatically cancel after a few more days.
Contacting the user would be a good first step if that is possible, but if their email ID doesn’t exist that would usually prevent them from being able to authenticate an account unless you disabled that.
If you have a hosted site with discoursemail the team will be able to help you with that, but is this on a different kind of server with other mail provider?