There's a general rule in most places that a person has an implied ability to access your property in order to announce their presence to the first reasonable point (eg knock on the front door, buzz an intercom - but not come through a locked gate with an intercom to knock your door). This general rule can be excluded so long as you provide a clear intention of doing so. For example: signs saying 'no doorknockers', or 'do not enter, beware of dog'.