cancelling the regen/powers of a subordinate who is connected to Imu through a limited seal, is not the same as actually canceling out Imu´s powers in person. We already saw Rocks vs Garling and Rocks vs Imu. It´s not really complicated
Shanks can´t do anything to Imu, he stole the Nika fruit for a reason. He goes to Rayleigh and Gaban to tell them about Luffy, not himself or the Haki he´s gained, for a reason