Pending can be removed by changing the Status of the pending in question.
On the pendings page, scroll to the pending in question
Using the Status dropdown, choose the Won, Lost, or Error Status (NOTE: You should ONLY use Error if it is a Duplicate or a mistake has been made)
Click Save at the bottom of the page
An explanation of Pending Statuses:
Pending = This is an active pending
Won = If an order has been received (the order amount may or may not match the pending amount)
Lost = If the order was not received, client budget was cancelled etc.
Error/Not Submitting = This is used if a pending was entered by accident, is a duplicate or if it is tied to an avail that you wont be submitting for (if Avails are enabled for your stations)