I want to disabled themes on a site collection in SharePoint 2010 and am not sure how to go about doing so.
Well, it will depend on a couple details.
If you're wanting to remove the permissions for all your users, you can do the following, via Central Administration.
Alternatively, if you're wanting to do this programatically depending on if you could write a feature stapler, that disables the feature 'Enhanced Theming', which has the GUID '068bc832-4951-11dc-8314-0800200c9a66', and even remove the managed themes altogether.
I've detailed all this in my blog article on removing/disabling themes, but this should be able to get you started.