I've always done it manually here how I do it.. Go into the data folder and find the ServerRewards folder and find reward_data and open. All the way at the bottom of the json file look for

"kits": {},
"commands": {   ect..

in there I was able to remove the command line. I then reloaded the plugin but first I did a json check before saving and reloading. 

Hope this helps in any way.