my macbook pro was stolen, and i want to block it by the serial number! how can i do that!

If your Mac is lost or stolen - Apple Support

You cannot track it using the serial number, there is no way to do this.

If you had Find My Mac turned on, you can use the Apple ID and Password to log into iCloud.com and track it.

Macs cannot be locked with Activation Lock however. You can lock it with a passcode, but that is easily bypassed by booting the Mac into Recovery and simply reinstalling macOS.