have iphone 3gs how can i block a phone

Contact your carrier to see if they offer call blocking, some do, some don't. There is no setting on an iPhone to block calls.

If your 3GS is not Jailbroken then you must be with Att. The only way to block a # is to add parental controls to your account thru ATT. I believe it's about 5$ a month. Your allowed to block a set number of phone # but I'm not sure how many. Hope this helps