Replacing a light kit on a ceiling fan with a cap?
The major problem with this are finding those caps. Wherever that fan was purchased, I'd be surprised if they sold let alone had any caps... I wish you the best of luck in your search though 

I've done many fan installs for people and have those instances too, where a person just wanted that light kit removed and the cap replaced. I got lucky a couple of times where people had their original caps.

The other times when there was no hope finding caps I've modified the light kit by removing the lights in them and capping off the holes left behind with flat head or round head carriage bolts, with nuts behind them. Painted to match the repaired area, of course.

Not too bad of a look, I believe.