S.S.S. (Steam Support Sux)

Late, and incorrect reply. Typical for Steam Support.

And by incorrect, I mean that achievements can be modified if you're willing to break the Terms of Service and technically run the risk of getting your account blocked, via Steam Achievement Manager.

If illiclt third-party tools can do it, Valve only cannot do it because they are too lazy to care.

Or it goes against company policy

You got a relevant response to your minor issue within 3 months. That's actually pretty good for Steam Support.

someone stealed my account , and steam support recovered for me, so i think sometimes works fine, but was only once,so take your chances :c

This one time, I don't even know how, I got my account hijacked and opened a ticket providing all the proof that I'm the real owner. Took maybe 2 weeks before I got an answer.
It was during summer and turned out that Gabe had took most of hes employees to Hawaii for a vacation. Not even mad.

You can remove them yourself with SAM, but if you unlocked one achievement in a game, it will be counted in your achievement average, even if you remove the game from your library.

What you expect? Its the SS ;) (small joke from a german idiot) :D