How can I keep Quicken (Windows) from automatically adding new stocks to the Watchlist?

I have never seen an option to decide if a new security should to the watch list or not.
And as such don't know any wat to prevent it.

One thought is Quicken might not put it on the watch list if the security is created while downloading, but I'm not sure about that.

But what determines what will show up in the portfolio isn't if a security is on the watch list or not.  Well unless of course you are actually looking at the watch list.
What determines when you stop seeing them is either you customizing that view and removing them from the security list or if all the shares have been sold if you have the Options -> Show closed lots, deselected.