If you're referring to the "Promote your other products - OPTIONAL" list of products on the final screen of each product's settings, that feature has actually been disabled pending renovations to related parts of our system.
In case that feature gets reactivated, you can de-select any product you're already selected (highlighted) in that list by holding the Ctrl key (on PC) or Command (on Mac) when you click a product you want to deselect (un-highlight). Products in that list which are unselected would have no effect; that box just lists all of your products, so you could pick which one(s) you wanted to cross-promote (when that feature was actually functional).
