Limit selection from other modeler

How do we make a UML object - an attribute, a Data Type, an enumeration - unselectable as far as being assinged to a Type Property of an Attribute?

We have tried:

1. Making the object visibility private compared to making it a package.

2. Making the object visibility protected compared to making it private.

3. Marking the abstract property TRUE (checkboxing it).

None of these marking methods appear to milit its assignment to a Type property.

We are trying to limit what a person composing message data classes can pick or assign, and reducing what objects are allowed to be assigned would be useful.