By its very nature, you can't really stop a DDOS attack. Not with consumer grade equipment, anyway.
Imagine a long line of people outside your door. Each ringing the doorbell for a couple of seconds, then moving out of the way. Now imagine that there are thousands of people in that line.
The only way you can make them stop is to:
1. Not draw their attention in the first place. They were all sent by one guy.
2. Call the 'cops' on them. In this case, your ISP and Steam.
3. Wait a day or two. They'll get bored and move on