Can an employer that once had designated smoke areas suddenly change the rules and have no smoking areas, ban it from the property, and fire smokers?

Yes, an employer may freely do this. Employers set workplace rules, and may change them at will--they may absolutely ban smoking at any time. Furthermore, since smoking is not protected under the law, the employer could terminate any smokers it wished to, so long as:

1) The smokers did not have employment contracts protecting their employment; or

2) The termination of smokers was not a "smokescreen" for illegal discrmination, such as against employees on the basis of race, sex, religion, age over 40, or disability--for example, if all the "smokers" fired are African American, or aged 50+, then there may be illegal discrimination going on.