The subscriptiopn model of dominion games is not "recurring". That means whatever you did/paid in the past, no additional payment will be forthcoming at some later time, unless you buy/extend a subscription yourself. So your subscription cannot be "cancelled" in that sense.
If you have a subscription running, but no longer want to use it, then you may be able to get a refund by the site owner for the remaining time. Note that this is based on goodwill of the owner.
If you just want to stop using Dominion Online, you can just walk away and do nothing. Some players prefer their account to be actually deleted, so I can do that as well, if you want.
If you want to do anything except to simply walk away, please say which option it is you want to be done!