There's no way in rejecting someone yet not hurting them at
all...
Agreed with Ah Ma, that if you don't like, make known to the person
early don't drag cos the longer you drag the more hurting it
gets.
One way I can think of is to distance yourself from the person
slowly, when ask you out... don't go, show less interest in going
out with the person, talk less with the person. If the person can
feel the signal then things will be just fine without even having
to reject cos the person won't even ask liao.