How to disable mysite for a particular site collection (SP 2007)? (disabling personal site through ssp affects all the site collections within that web application)
You'll need to use a custom master page for that site collection and remove the link to MySites. To do this, you'll need to turn on the publishing infrastructure feature in the Site Collection features. This will open up the master page gallery. Then using Sharepoint designer, you can create a new master page that hides this control, save it, check it in, approve it, then apply it to the site and the subsites and the link won't be there.
My Site isn't attached to any site collection, it's a site collection of it's own and surfaced through the My Site links on a master page. So technically you can't disable it for any particular site collection.
You have two options:
You can remove the My Site link by disabling/removing it in the master page for that particular site collection (PirateEric's answer). Although all you're doing is hiding the link to a site that users can get to from any other site collection in the farm (or if they know the name of the url like http://mysite or whatever you call it).
You can turn off my site access in Central Admin SSP by removing the permissions for users to view/create/etc. My Sites. This will leave any sites intact but not allow any user to go to the url (they'll just get an access denied error).