It seems some you can, and some you can't. I know I have gotten quests, then deleted them because I just wanted to see what they offered or what the quest entailed, and then gotten them again later.
However, in Nektulos, the new quest guy (Dark Elf NE of Commonlands, near spiders/treants) gave me a bunch of armor quests, I got them all and realized my journal was full after that - so I deleted a few that seemed to tedious to bother with at the moment, now that I have room and figured I wanted to finish those - he doesnt offer them to me. =/
Oh well.