If you choose to answer a door knock/bell to an unknown person and once you've identified it's someone you do not wish to deal with (even someone delivering a parcel to an address which YOU may have given), it's quite simple to be polite in saying "I'm not interested" and close the door.

As an occupier of a property, you have every right to do this to police, flower deliverers, parcel deliverers or salesmen.  The action such visitors take next will depend on the reason for their visit in the first place.