I believe the post they were referring to is this one, posted 24 minutes prior, where they say:
The idea is that if you give the username whenever you call getnewaddress, you can get the user's total received across all their addresses using the "bylabel" functions. You can freely change their address without worrying about tracking all their old addresses.
A good way to automate changing the user's receiving address: just before displaying their current address, check if it has been used to receive anything, if it has then replace it with a new one:
// Get a new address whenever the current one has received anything
if (strAddr == "" || getreceivedbyaddress(strAddr) > 0)
strAddr = getnewaddress(strUsername); // Label the address with username
Display(strAddr); // Display their current receiving address
I think the description and code are self-explanatory: check if the address has been used, and if so, generate a new one.