Just contact Steam support to have the DLC removed from your account. Why is this so difficult to understand?
...And you have dealt with steam support, right? I currently have an outstanding ticket that is over a month old. Steam has tried to close the ticket three times, with no fix offered, and still has no clue as to what to do to fix the issue.
I have, BTW, opened a ticket on this issue, as I also feel that I shouldn't have to re-download the DLC content in order to re-load my game. Unlike some of the yanks that populate this site, some of us in other parts of the world have limited options in our speed / size of pipes available, as well as our up-download monthly limits. 3 GB is one seventh of my allotment / month, and I am not happy to have to download that every time I want to re-install. I would rather pay a one-time fee for phisical media if they cannot make it so that a DLC can be linked / unlinked from an account. I hope that it is not the same with the steamworks mods as well, I may try ten mods and end up using one. I'd hate to re-load my game and have to then re-load every mod that I'd tried, just because I'd tried them...
...Just did a quick count... I've tried 153 mods, and use (currently) 11, counting the two hi-res DLC's.