OldFatGuy: What are reasons for locking people out of Steam?? Is one lack of use???
The reasons for Steam bans are related to fraud. Just dont try anything stupid and you wont run into trouble. And as someone above me already mentioned, avoid using Paypal on Steam. There are a few reports of accounts getting locked due to problems with payment through Paypal. I dont know if these reports are actually true or not, but i wouldnt risk it. I trust Valves security measures enough, no need to use Paypal for me.
One thing about Paypal that is really true is that you should NEVER EVER open a dispute on something you bought on Steam. This really gets your account suspended till you fix the issue, which will take quite a long time, sine Steams custommers services is very slow.
Oh, and never "farm" those "gift points" (or whatever they call it) they give out on big sales (summer and christmas). There were a few people who got all their accounts banned for making new accounts just to get these points.
And dont try to bypass region restrictions using a proxy. Thats pretty obvious.
There are also reports of people who got banned for registering a retail key from another region. You know, those russian sites which sell cheap retail cd-keys. Stay away from those.