Unsupported: Use at your own risk
These three simple lines allow you to clear the Run History.
You can use this when admin client does not allow you to clear the history through the UI. I've seen this a couple of times when History is huge.
TRUNCATE TABLE FIMSynchronizationService.dbo.mms_run_history;
TRUNCATE TABLE FIMSynchronizationService.dbo.mms_step_history;
TRUNCATE TABLE FIMSynchronizationService.dbo.mms_step_object_details;