You can clear the cache by stopping the Web Proxy Service (in the ISA Server Management tool) then deleting the urlcache folder or deleting the dir1.cdat file located at x:\urlcache, the location of which will be specified in the cache configuration section.
Restart the Web Proxy service and the urlcache folder and it’s contents will be recreated (albeit empty)
Restart the Web Proxy service and the urlcache folder and it’s contents will be recreated (albeit empty)
p.s this assumes that you have ISA server 2000.