Long running statements make Toad 12.8 appear as 'Not Responding' when task continues in the background. It is therefore not possible to cancel the statement.
Please execute the statement using F9.
Script execution does not execute in a background thread and since this one statement takes a long time to execute without returning anything, Windows thinks it has become unresponsive.
F9 does execute in a background thread (regardless of the threaded queries option) so Windows should never flag it. For single statements using F9 is the best method to run them.