Once set in Roma, it seems impossible to disable the setting of a list of values for an attribute.
Here are the steps to reproduce it :
1. Create a new customization
2. Go to module > core > abbr (or any other element) > change attributes > set ana (or any other attribute)
3. In the field "list of values", write for example: test1,test2. Make no other change, save it.
4. Reopen ana. First, you can see that "closed list" has been switched to "yes", wich is not what we wanted. It seems impossible to set an open list of values with Roma.
5. Clear the field "list of values" and leave it blank. Save it.
6. Reopen ana. Nothing has been changed, there is still our list of values in the field.
I have tested this several times, it always happens. I haven't seen it reported before.