Workspace Owners/Admins are able to disable specific reward types.
For example, if a company only wants to allow custom company rewards, they can disable the others and only custom company will show in the Rewards tab.
Disabling Reward Types/Categories
Open the Web App.
Go to Channel Settings.
Then Rewards tab.
Under 'Categories' disable the desired reward type(s)/categories.
Note: You can't disable custom rewards, instead you will need to delete the reward.