Hi,
I will let someone else advise you on the ISA configuration portion of your question. To disable MySites:
Go to the Central Administration
Click on your SSP (If you have more than one SSP, select the one that is running the MySites functionality under "User Profiles and My Sites" click Personalization Services Permissions)
Select the group you want to limit the functionality for. More than likely you will just have NTAuthority\Authenticated Users or Everyone.
On the next page you will see a list of checkboxes,
Tick the check-box to disable MySites uncheck "Create Personal Site"
Tick the check-box to disable MyLinks uncheck "Use Personal Features"
Hopefully that helps,
Sincerely,
Karl