Kinda. It depends what you're trying to do - for example if you want to replace the handler for a DOM event, you can do that easily enough, but other things aren't possible like simply replacing a function. And you can't touch code that takes place inside someone else's closure...
Yes you can do this, but unless they have the bit of script you want to block in a separate file, then you're likely to be blocking all the other code which is often required to make the site work!
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