I know exactly what you mean. I think I have about 2 dozen on my list now. Every bloody person I glance at in-game runs up and wants me to do something.
As for declining... you CAN - just escape out of the dialog. I can't test this at the moment, but I believe that choosing the "not right now" dialog still adds the quest.
I have the same.. No matter how hard I try to concentrate on let's say main quest, ten minutes later I've done a bit of the thief's guild, a few random favors, going for a mission with Companions and got 5 new quest at the side. I'm happy there are so much to do, I just need better self-control.
I tried to do a little of everything. For example I would have an active main quest, one of the guild quests, a word of power or hold quest and then some time doing the in town stuff. Once I'd done a quest from each I'd start the cycle again.
Doing it this way I found that I wasn't rushing through a given questline, it kept it varied so it wasnt just one dungeon or ruin after another and levelled my character up in a balanced way.
I tried to just have one quest marker active at once as well otherwise u spend half the time checking the map to see if your heading towards the right marker