i think just be honest in your response, or you could just keep saying no without owing an explanation ... Repeat it until it sinks in. I find it works when people are trying to squeeze a different answer from me or think they are in a position to negotiate and often times, a simple no (albeit repeatedly) does the trick.
From what you have described it sounds like these men have special needs which I gather is because they are not able to take care of themselves. They sound a little child-like and I personally don't think they are scheming to take advantage of you. it could just be that they are so sheltered that they don't understand boundaries and social norms. in saying this I believe it is healthy for you to setup clear boundaries and only help as long as you feel good about it otherwise you may end up not only resenting them but also yourself for being their doormat.