Restrict Affect and Fix version

[James Brown]

How can I implement the following restrictions on the Affects and Fix Version fields.

1) Not allow fill in versions so they can only pick the ones that have been created in the version list

2)Prevent users from picking more than one version?

[Steven F Behnke]

1. Project Admins always have the ability to create Versions inline. You cannot restrict this.
2. You could only do this with a Transition Validator during a workflow action.