I have several dropdowns in my customized work item layout which get allowed values from global lists.

I note that I can type a load of garbage into the dropdowns at runtime - although the system won't let a user save a value that's not in the global list, it would be nicer to stop the user being able to type in at all. Of course, ReadOnly just means that no dropdown is displayed.

Any way I can stop users typing into dropdowns?

I don't think you can stop typing into dropdowns. But as you said, users can't save or publish the invalid value, also users will get an error message if they try to do that.

Sometimes the dropdown list can be very long and hard to find the one you need, so I think typing is useful in this situation.

There is no simple way to do this I think.

The possible solution which comes to my mind is creating your own custom control for this, you will need TFS SP1, and here you have a really good instructions step-by-step on developing custom controls:

http://blogs.msdn.com/narend/archive/2006/10/02/How-to-use-Custom-Controls-in-Work-Item-Form.aspx