[iMore.com]

If you frequently receive iMessages from people not stored in your address book, you can quickly filter them out.
A lot of us depend on iMessage to communicate with friends, family, clients, and colleagues. That means not every message we receive is attached to someone in our contacts. If you keep finding yourself buried in tons of iMessage threads, some less important than others, here's how to filter out messages from those you don't have saved.

Full story from the iMore Blog...

[Hendri Hendri]

Can we filter sms from unknown number as well ?