Level 1

How can I delete all activity for one specific day?

Level 15

Reports and accounting

One transaction at a time, bring each one up, use the more menu link at the bottom select delete

Level 10

Reports and accounting

If you added them from Banking>For Review, then you can go to banking>In Quickbooks, check off the transactions you want to delete, and click Batch Actions>Undo.  This deletes them from the register, and puts them back in For Review, where you can either re-add them, or Exclude them