Prevent users from being able to choose a facilitator type other than “Training supplier”

In the “Create an offering” task, in the “Offering details” section, there is a “Facilitator type” field. In this field, 2 values “Instructor” and “Training supplier” can be displayed. The customer wants to prevent users from being able to choose a facilitator type other than “Training supplier”.
To meet this need, I've tried to make the “Instructor” lookup enabled, but as it's a standard field and the lookups are defined by Oracle by default, we can't make the “Instructor” value enabled. Is there any other way of preventing users from choosing a value other than “Training supplier” please ?

Thank you

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