I simply don't answer. Eventually, they give up and go away, or I'll put my phone on mute and shout at them "I'M WORKING GTFO" and then cheerfully return to my customer. If it's a delivery, it's just a quick courtesy knock to let you know they've been there, unless they need a signature. Usually a Sign on the door "please do not knock on this door, I am working and I will not answer" solves the problem. Make sure you use Clearly printed block letters though.

The phone sales I handled in a different way, I had one company call me 5 times before noon... 5 times in one morning is a bit much. They called twice on my lunch break. I answered the third call "Not interested, don't call again" Fourth Call "I'm going to say this one more time, I'm not interested in your product, do not call me again" fifth call thirty minutes later "ok, Here's the deal. I"m only going to say this once. The next person to call me from your company is going to get nothing but swear words and a hang up. Do not call me again." I've not received a call from them since.