I don't like websites that spawn pop ups on my computer. I use software that blocks them. How can I stop the annoying steam popups asking me to buy their products?

Uncheck "Notify me (with Steam instant messages) about additions or changes to my games, new releases, and upcoming releases." in the menu Steam/Settings/Interface.

I love that pop-up, I get most of my 50-75% off deals through that bad boy.

Thanks.