You can quite happily delete data from the steam apps folder and steam is quite good at making it fairly obvious what it is to do with, but obviously bear in mind that games which use the half life 2 engine will require that to remain to function.
Also another thing, to prevent you having to redownload the game is to back the relevant files up to another hard drive, then you can just copy them back into place when you want to start playing again.
Also, if you need to reformat the hard drive, steam will quite happily be backed up and reassert itself on the target computer by simply running the steam.exe. Also if you do reinstall steam from scratch it will happily take a backed up copy of the steamapps folder and make the contained games available to that copy of steam (but obviously only if the logged in user owns them)
So whoever has informed you that the only way to deal with these problems is to reinstall steam is mistaken (happily)