Bentonite powder was what we use to use. Make sure what ever you use you clean the ropes and sheave grooves.

Changing the accel and decel rate may serve as a temporary solution. Other things you may look at is the amount of rope that is making contact on the sheave, the rope groove wear and the hoist cable diameter. All I'm sure you have already considered...

Just a suggestion if the ropes were lubricated and it has soaked in, would it make more sense to replace the ropes?