If by harassing, you mean sending you letters, the software prints those. It does it for all bills under some threshold(I forget the amount now). Humans are barely involved in that process.

If you have literal IRS agents showing up wanting their $300, I'm seriously surprised. I'd recommend you verify they really are IRS agents and not scammers.

You should just pay the $300 and move on with life though. You will spend more than $300 of your time/energy trying to convince the IRS you don't owe it(assuming you feel that way).

In general the IRS is very forgiving and tolerant up to a point, but eventually you get put on their naughty list. Much like a sleeping giant, they are happy to snooze away for a decade or so most of the time, but eventually they wake up and get angry. You never want the IRS actually angry with you, it almost never works out in your favor.