Yes, it is possible to delete contacts from your SendinBlue account. To permanently delete a contact or multiple contacts from your database, go to the "Contacts" dashboard within your SendinBlue account.
In the first drop-down menu titled "All Contacts", select a list or search for the contact(s) within your full list. You can add a search condition by clicking on "Advanced Search & Segmentation". For example, you can select "Email" "similar to" "xx@gmx.de" and click on "Search".
Check the box to the left of "Email Address", and then click on "Actions" and "Delete permanently".