By selecting passive skills which reduce your damage I was able to cause all of my projectile (and spells with projectiles) damage to do negative numbers. One would expect that damage could not be reduced below a value of 1.

Build focused on DOT damage so negative modifiers to other types of damage should have been an acceptable loss.
Selected all DOT amplifiers under cabolist tree that included reduction in other types of damage. -30%, -15%, -15%, -10%
Selected dual projectiles under Ranger tree that reduced damage for projectiles/spells by -60%

Tool tips showed negative numbers for damage for skills. Because of this negative damage, No DOTs get applied to targets.