Log in to Directadmin. If you are not sure how to get here, you can look in this guide.

Then navigate to E-mail Manager -> E-mail Accounts

Then select which account you want to purge and press purge

I the top field you choose which folder you want to purge for the account you selected:

I the field below you choose if you want to delete all emails in the folder or if you want to delete emails from a certain time ago:

Then press purge