You can delete a closed Pool, but this removes the information from the database and that is not advised.

A better approach is to limit the Pools you see in the Pool data grid to only the active Pools. 
  1. Go to the Pool record for each of the inactive Pools and enter a Closed Date. 
  2. Create a data grid in the Pool supertab  that selects on the Closed Date.  Leave the from and through empty or blank. 
  3. If you wish this grid you be your default grid, create it as the "User Default View".