The process very simple. First, go to Settings of the chosen key in the application, and tap Invite and manage guests. Click Edit in the top right corner, and select the guest you would like to revoke access from (you can select more than one guest).
Simply tap the revoke access button and the access will be revoked from the selected guest(s).
Sometimes you need to activate these changes on the Nold Open device too. If you go back to your key list, you should see the status of your devices on your keys after you revoked access from someone. If not, just refresh the key list(pull-to-refresh) and tap on the keys that needs a confirmation. Keep in mind that you need to be in close range of your Nold Open device to do this.