I'd be glad to help you delete your 2017 transactions showing up on your reconciliation screen.
To delete the transactions successfully, you have to open each transaction. Once done, click on More on the transaction/s page, select Delete, then click on Yes to confirm the deletion. On the other hand, if the transactions are downloaded from the bank, make sure to Exclude them and not Add.
You can see attached screenshots for additional reference.
To check if the transaction/s is successfully deleted, you can check the Audit Log.