I am sick of it. Whenever I want to see what free games are on GOG I get face full of ads (a.k.a. demos). I can filter out DLCs and owned games. But I can't filter out demos (which are, despite not costing anything, DEFINITELY NOT FREE), Prologues and other crap like that, leaving only actually free stuff.

Fix it, GOG.

So first filter by free games. The scroll down to "tags" and search for "demo". When demo appears below the search box, hover over it and an eye icon will appear. Click the eye icon to hide games with that tag.

Despite not costing anything, they cost everything. Isn't it obvious? :P

If you look in the mirror and say the word 'demo' seven times, freebies cost the ultimate price

Coming to cinemas this summerrrrrrrrr

Thanks. I didn't know it's possible.
However I agree with OP that the filter should be more exposed and also it should be possible also to easily hide all demos in a user's library. It's well known problem and a lot of people complain about it – yet they do nothing about it for years.

Demos are actually free, so one can filter by cost and remove them altogether if they are that annoying.

Oh, wait, did you mean in one's own library? Because anyone is able to hide/unhide whatever they wish, if that's the case.

That being said, it wouldn't be impossible for GOG to add another product type ("DEMO") for said entries and a filter.

You need to do it manually for every single title, which is not very convenient in my opinion. I guess most of people do not actively "manage" their libraries and simply want such feature to do it for them.

GOG still exists? *rages to the ends of the earth*

While this is the way to go about it, the problem is GOG haven't bothered to tag many demos as such.

Well, what about demos with unique content? Usually called prologue or some such nowadays. Pretty good way not to spoil a story if the first part of the game is unsuitable as a demo.