You can do it at Steemworld very easily. Just get onto your Steemworld Stats acct and click on the Delegations tab n the left column. Then click on the “incoming” tab. This will bring up the list of everyone who delegated to you. On the right of that list you will find another tab to remove the delegation. You will need to use Steem connect or Keychain to authorize the transaction. This will return the delegate Steem back to them.