It seems that if you select a UI type editor on a property of a custom activity, there's no way to remove it later on. If you go into the Property definition window, and just clear the UI Type Editor field and save the changes, the type editor is not removed from code (though if you change it to another type it works).
The only way to remove it seems to go into the code and manually remove the appropriate attribute out of the property definition.
BTW, while on the topic, doesn't anyone find it weird (that is, ugly) that the custom activity designer (for a code based activity) generates the code for the custom properties into the main .CS file for the activity and not in the .Designer.CS file? After all, that's why we have partial classes and all: to get the designers to not mess around with our custom code files....