I'm going to ignore a guy that likes me
Is there a reason you want to ignore them?
I'm not ready for a relationship right now
It's okay to not want to return someone's feelings if you aren't interested in them. However ignoring them is rude and it may hurt their feelings. Try explaining your actual reasons to them, they may be understanding of your situation.
I'll let him suffer and make it up to me later.
It is rude to ignore people if they are trying to talk to you. You should be honest with how you feel and if they don't respect that you can ignore them.