You can delete the 300 imported transactions by excluding them from the For Review tab, @racquel79. It keeps your financial records accurate in the program. I'll guide you how. 

1. Go to Banking from the left menu. 
2. Select For Review.
3. Check the box next to each transaction that shows as a credit for the 300 ones. 
4. Click the Batch actions drop-down.
5. Choose Exclude Selected.

The screenshot below shows you the last four steps. For detailed instructions, check out this article: Exclude Expenses From Downloaded Bank Transactions. 

After that, the transactions will move to the Excluded tab. They won’t appear on your account registers or reports. 

Moreover, you can assign, edit, categorise, and add your transactions. Doing so will help you ensure they have correct categories before reconciling the account. To learn more about this process, visit this article: Categorising Transactions. 

I'm just a comment away if you need anything else. Enjoy the rest of your day, @racquel79.