I have not understood if you use Captive Portal authentication or WPA/WPA2 with 802.1x.
In the first case the possibility to cache the passwords is a feature of the web browser and you are able to decide if you want to use it. In any case, by using the tools of the browser you can remove any cached password.
In the latter case, I don’t know if it is possible to disable the MSChapv2 password caching in the PEAP.
If it is not possible you shouldn’t use the Windows XP’s integrated supplicant, but install another one that supports PEAP authentication.