I am of the opinion that employers should not be able to permanently ban anyone, hired via at-will employment doctrine, ceteris paribus.
As a check and balance of rights between the employer and employee; I think the employer should only have recourse to banning someone from employment, from that specific company, for a specified length of time.
The caveat (for the employer) should be that of stating a specific length of time; and by implication the specific length of employment.
As an example, a two year ban on employment with that employer (should the employment not work out) should require (the equivalent of) a two year employment obligation; if, and only if, they want recourse to a two year ban on any specific individual.

