Yes, but by the time you did it you already wasted time checking out that game. And it will happen again, and again, and again... on every single sale it is presented. Add it up for several games at a huge sale like the current one and you're looking at a waste of hours per year.

Unless you have an astounding memory that manages to remember for every game you don't own which ones you decided weren't to your liking and which ones you decided you would buy at a later time. If you do major props to you, but my (somewhat good) memory reached its limit by the time Gog's catalogue grew up to 2000 entries.

I get the OP's need and reasoning. It's basically the same reason a "Hide DLCs' flag was added. Ignoring games you don't want will help you find and buy games you want, which is a win for everybody including Gog.

BTW, according to Gonchi's description it seems like Steam's system is exactly what we need. It even helps avoiding games from those publishers that release a game here and never update. How many times we got a new release that looked interesting only for someone to say "Ugh, publisher X? Beware everyone, unless you like your games outdated"