It seems impossible to delete your account. We could not find any information on account termination.
Perhaps try a request by mail (firstname.lastname@example.org).
Some interesting rule from their ToS:
“If you find that the information is not accurate, complete or updated, then
you may request to correct, amend or delete such information. We may refuse to comply with your request for various
Besides, they may retain your personal information, as also stated there.