I'm thinking of ignoring somebody who I know is suicidal.
What makes you think ignoring a suicidal person is a good idea?
I think talking to them about it would only make them think about it more often.
I don't think you showing concern for a suicidal person would make them more suicidal. Ignoring them might make them feel even more alone at a time when they need a kind person to help them. See if they need help contacting a medical professional to help them.
I refuse to help anyone who is weak.
I don't think it's very nice to refuse to help someone because you think they're weak. Everyone needs some help now and then, and by helping them you'd be doing them a great service.