ich need host to survive. if u can leave your tank fishless for 6 weeks, your tank will be ichless. it is okie to leave shrimps and other invert such as starfish inside as ich do not attck them and had no capability to attach themselves on their bodies unlike fishes.
UV will help to control, however it can only help to eradicate those on free swimming parasite only.
if u have fishes in the tank and observe for 6 weeks, there are still possibilities that fish are still affected by ich but beyond our Na ked eyes, so it is not a good assumption to say that it is ichless if u still have fishes inside