To delete create or delete aliases you need to follow this steps;
1) Log into https://account.riseup.net
2) Click on the "Mail settings"
3) Under the "General" tab you will see an aliases box. To add one write it there, one per line, in the format of firstname.lastname@example.org. If you want to delete one, just remove it.
4) Click the "Save" button. The alias should be working now.