How can I block a number in my iMessage?

I don't think you can

You can't do it.

There is currently no way to do this with iMessage. You can turn off iMessage, and, if your carrier provides a Block Number feature, you can use that so it will prevent them from sending you calls and SMS/MMS.

This is such a serious loophole. I don't want to have to turn off iMessages for everyone for this one person. Apple needs to fix this and allow numbers to be blocked.

I agree this is a serious and potentially dangerous oversight on apple's part. I am currently being stalked and harassed by another iphone user and they have been sending me enormous amounts of texts via imessage and cloud email. i have fixed the email problem but still am recieving sometimes 40 texts at a time. its been going on for several months now and the aggression within these texts is escelating.. i have friends and family who need access to my imessage feature so i am afraid to turn it off, but i dont know what else to do? turning off imessage only isolates me further and causes me further vulnerability. ,

any advice will be greatly appreciated tired of hiding.

Report to the police, harassment is a crime.

Save the messages. The police will use them in their investigation. Harassment using phones, or other communication devices is a crime. Threats of violence are a separate and more serious crime.

Don't delay. Call your police department now.

Unfortunately Apple seems to have missed this problem. It's a serious one which needs their full attention. Without the ability to block undesirable senders, iMessage may one day become unusable.

It will be included in iOS 7 to be released in the fall.

Has this been confirmed?

Yes, it was in the slide on additional features during the keynote presentation on iOS 7. It also includes to block phone calls, texts and facetime calls.