We have just added a new housekeeping function which lets you delete old job applicants before a certain date.
Because we have a lot of customers in the EU region, this is an important requirement for GDPR compliance, but other countries have similar limitations on keeping personal information for non employees past a certain timeframe.
If you go to Setup -> Tools -> Advanced, you should see the following option on there:
Clicking on the button will ask you for a date, and when you enter a date and click the red 'Delete Applicants' button, this will bulk remove any applicants who was added to the system before that date.
Caution: This will remove all the job applications and attachments belonging to that applicant as well. This process is NOT recoverable, so please use caution when you run this process.