For the simplest possible example, ask any Noscript extension user.

For the more advanced things, like identifying the part of Javascript code that brings e.g. 'the fancy eating 40% of the CPU scrollbar' user want to disable, some automation is needed.

I am a NoScript user. I use it to block all JS by default. If a web site won't work properly that way, and the site is somehow really important, I'll work out what the minimum scripts necessary to let the site do what I need is, and just allow those. If the site isn't somehow critical, I just don't use the site.

I have to do it that way because it's very difficult to determine which scripts are bad and which aren't without actually reviewing the JS itself. In my view, "bad" usually means that the script is phoning home in some way.