If you wish to cancel your delegation all you need to do is simply set the amount delegated to 0 in the same way which you delegated.
If you're doing this via SteemConnect simply comment the following URL:
https://v2.steemconnect.com/sign/delegateVestingShares?delegator=<youraccount>&delegatee=<theiraccount>&vesting_shares=0%20VESTS
Within this you will want to add your account, you should do this by replacing <youraccount>, then you want to add the account you wish to to remove the delegation from by replacing <theiraccount> and the finally the amount you wish to now delegate. This is already set to zero.
Remember it will take 7 days for you Steem Power to be available for you to vote with again.