If you're using phpMyAdmin, it's on the Operations pages, Empty The Table (TRUNCATE). Be careful not to delete it.
Another thing I think would work would be to go to Server Settings and Optimization Options in the admincp settings, and set Cached Posts Lifespan to 0 (after first noticing what it's set to and remembering it), then under the Scheduled Task Manager, run the Hourly Cleanup #2 task. Then set the Cached Posts Lifespan back to it's original value.
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