How do I remove apps that can't be uninstalled?

Clear data then disable. The app can't run (and hence can't accumulate data or updates) and as the app itself is in the system partition it's not using any user space - if you could uninstall it (which requires root) you'd have no more space than the recipe above gives you.

And that other space may be your cache partition. It's hard to say since its not clear what you are counting in your total (were those system apps part of the total? They shouldn't be, because they live in a different partition, but their data should be). You probably need to keep 200MB free as working space, which is why you have a problem with updates.

 

To really remove those apps you will have to Root your phone. Rooting procedures vary by device so check over at XDA Developers or maybe here to find the Root method for your phone. Because you're running Ice Cream Sandwich you have an option which is no longer available that allows you to Hide unwanted apps. Check out this article over at makeuseof for more info.

HTH