Losing access over a purchase dispute doesn't happen anymore. Valve changed it late last year and now the only thing you lose is the ability to purchase anything on the store, activate new cd keys, and play online.
Not quite. Generally, you'll be hit with these restrictions:
![support.png](https://dl.dropbox.com/u/21565521/steam/support.png)
However, if Valve suspects the account has been used by somebody else, Steam Community access will also be disabled:
![support2.png](https://dl.dropbox.com/u/21565521/sekrit/support2.png)
(Images taken from SPUF.)