Yes I think I am going to get tougher and be ruder - I am obviously being too polite!
Had another company today - not ADT but one of those companies who want to advise you on making improvements to your house - they were even more pushy than ADT but I was firmer and shut the door (the guy didn't do himself any favours by giving my front steps a kick and telling me they were falling apart! Incredibly rude). Actually think I am going to get a 'no cold callers' sign as I am getting so many of them now.
It must be really distressing for older or more vulnerable people.