Possible to 'not' install owned dlc on steam?

You can't stop it from being installed, to my knowledge, but depending on the game, it should be possible to prevent it taking effect by removing the right files.

I had this same problem. I bought all the DLC for sleeping dogs. But half of it eliminated the point of leveling up your stats, as it gave you all the best shit from the beginning.
Other dlc also puts you straight to level 8 in every category (the max is 10)

There isn't, though it has been requested, partly because of DLCs like the ones mentioned or that make it harder (BoI), and some because not all users can spare the extra data usage to download the game and expansions at the same time, but still want the game.

try google it. some DLC can be disabled with certain technique. for example :
binding of ishaac dlc : using launcher
sleeping dogs dlc : rename/delete certain files

I once used a mod what game?