Deactivating needless properties in Django Admin
Another way to solve this is by adding javascript that disable the "other" fields when you set one. You can then enforce this in the form/model validation.
But I should say that I think the best way to deal with this, if it can be applied to your problem, is the way PT114 propose.
We have three properties on my animal model:

dog_name 
cat_name 
monkey_name

One of them must be filled (no more! animal is a dog, a cat or a monkey) and if I set for example cat_name, I want dog_name and monkey_name to be deactivated (user shouldn't set more than one name).
Is it possible to set this in django admin?
This example is maybe stupid, but I tried to explain my intensions - deactivating needless properties.