Hi @Máté Kubicsek 

You cannot delete the account if it was previously used. Only exception if the account is used in a manual voucher and voucher is not updated to GL. Then go to voucher and change the account. You can try deleting. 

If the voucher is already updated to GL, there is no possibility to delete it. You can change valid to date if you don't want to allow users to use it.

Hope this helps