Deleted transactions are imported to your bank account again when a new sync is triggered because Business Finance Assistant needs to be consistent between the account balance you see on your bank portal and the one you see in Business Finance Assistant. Not having the same transactions in both places will cause your account's balance to be different from that of your online banking.
To switch off the bank sync so the deleted transactions do not reappear, please refer to the steps in this article: How do I switch off bank synchronisation?