1) If the original Salesforce user who installed and configured MassMailer is inactive, you have to reassign that license to a new Salesforce Admin user by going to MassMailer Setup -> Users

2) Once you have assigned a new MassMailer Admin user, now you should go to Salesforce Setup -> Scheduled Jobs -> Identify the following scheduled jobs and delete them all.

3) Now, go back to MassMailer Setup -> Finish -> click on the Finish button.