What's the best way to prevent unsolicited mail from being delivered?

How to opt out

I’m sorry you are going through this. I had a similar situation with a relative only she was writing checks to every organization that begged. Once she did it, the requests never stopped – many per day!
Since we could not stop the organizations from sending her requests we decided to cancel the checking account have her mail forwarded to us and we would pass on legitimate mail after filtering (and paying her bills ) .
Thank goodness she was not using the internet.

In practise, opting out is nearly impossible because so much junk mail isn’t from legitimate companies anyway. I would think @LuckyGuy‘s tactic of getting the mail redirected to yourself is probably the best bet.

@downtide Between that and the credit card application opt out and catalog opt out that I filled out, I cut my junk my junk mail to almost nothing.

@syz Credit card applications and catalogs are easy to stop. They were not the problem.
She was getting first class mail from organizations she supposedly had “relationships” with in the past. Really? NAACP, American Indian Foundation, Sheriff Deputy Fund, American Lung of XYZ County, etc. There were mailings from organizations with names that looked like real organizations. Multiple mailings from legit organizations advertising the annual spring fund raiser, the annual fall fund raiser, “This will be the only time we will contact you for our Annual Winter Fundraiser.” Slime.
It was best if I was the filter.

If all else fails, keep a waste basket next to the mail slot and toss each unwanted missive as it comes in. More trouble would be to write Return to Sender on each piece and let the mailman pick them up the next day.