No, its more an issue of removing them completely before building. The first time you build everything the post build copies the files up to web including config files for indexbuilders. So if you then change the build to leave out the mojoPortal.Featurres but the config file is still there it will cause an error because the dll is missing or expected methods are missing in the dll as in this case. Post Build events basically install features but there is no un-install other than manually deleting files. Its a little more difficult to remove a feature that is part of a set of features, like to remove forums you also need to remove the forum related files from mojoPortal.Features.UI/Setup/ProviderConfig/indexbuilders and /sitepredeletehandlers before you build so those files won't get copied up to the corresponding Web/Setup locations
You would probably want to edit the post build event to leave out the /Forums folder and would want to remove the mojoPortal.Features.UI/Setup/applications/fourms folder as well.
Hope it helps,
Joe