I use DK a lot and I have some pretty nice options to approach with.
You absolutely MUST use the Ftilt for approaching a spammy player. It'll eat any projectile alive provided it's timed right. DK, as I've noticed, is an anti-camper and anti-spammer because of his range with his Ftilt. Another good approach is his back air as long as you short hop it.
A good thing to do against a very defensive opponent is to approach them with a down grab into ftilt. This only works best at lower percents though (it does work at higher percents, but, you actually have to chase after them a little a ways). Try it, it works quite well a lot of the time (but, can be very predictable if you do it too much).
Am I forgetting anything else? =\