how can i block contacts on my iphone 3gs?

Only your carrier can block calls, this is true for all cell phones. So, contact your carrier to see if they offer call blocking. Some do, some don't. Otherwise, assign a silent ringtone to these contacts. The call will still come through, but your phone won't ring.

For blocing a particular contact from your iphone 3gs follow these steps:

First download and install backlist app on your iphone.

Go to settings.

Open backlist app.
Cick on plus symbol option.

Choose input manually.
Enter the contact name and phone number to block it.

Confirm the contact.

Finally touch save option.

You can ignore Patel's post, because the calls are NOT actually blocked. As I've already provided, only a carrier can block calls on any cell phone. There is no other way to block calls...none. You can accomplish the same thing by merely assigning a silent ringtone.

Additionally, you should ignore Patel's post as pretty much every blacklist app in the App Store gets terrible reviews. They are not worth your time or money. wjosten's advice is what you should follow.