The only real way you will be able to stop a speed hacker in-game is to voteban when you see them. The anti-cheat will have no affect what-so-ever on external programs (ie. Cheat Engine).
The only thing a little scriptkiddy needs to know is the proper stings and threads for speed in Garry's Mod, and programs like Cheat Engine would allow them to edit the variable.
Health on the other hand would not be hackable as it is lead not by GMod's main coding but by LUA. Unless you could find a way to change the gamemode settings at will. Which is impossible because the needed variables to edit on Serverside and the Client (Player) has no say at all in what they may be.
All you can do with said speedhacker would be to just report him and hope he gets a sufficient ban to let him think about choices such as hacking. If you have the Steam ID then you should be fine as long as the admins add his ID to the ban list.
The only real way to block these people from speed hacking would be the VAC, and apparently it sucks ass and is not doing its job.
I felt the need to help you out so I uploaded the demo file to Mediafire and Multiupload.
http://www.multiupload.com/AR862ZQVDKhttp://www.mediafire.com/?iledgoyyymwYou should replace the link in the main post with the link to the Mediafire one I provided.
