Well, I usually do not have that problem because everyone knows I am broke LOL! But, in the rare case, if someone does ask me, I always try to give something if I have it to give. I know how it feels to need financial help and there have been people who have helped me out in my time of need.
Also, when it comes to money, I don't do loans because 1) I don't have a whole lot of money to loan in the first place and 2) I just try to give what I can to help out without expectations of getting the money back.
If I did have it to loan and the same person kept asking, I would just tell them I do not have the money to lend. It hurts them in the long run to keep giving them money when they ask. It will get to where they expect it and will be angry at you if you don't give the money and that's no good at all.