You can advertise that being authorized to work in the U.S. is a precondition of employment.
The U.S. Office of Special Counsel for Immigration Related Unfair Employment Practices (OSC) stated in a technical bulletin on September 27, 2010, that:
...if an employer chooses not to employ persons who require sponsorship for an employment visa, such as an H-1B visa, the employer may state in its job postings that it will not sponsor applicants for work visas.
In summary, you can advertise that being authorized to work in the U.S. is a precondition of employment. List this information in a company policy, such as in your Employee Handbook, stating that you dont sponsor employment visas, even though you may have done so in the past.
Alternatively, if you do sponsor employment visas, include such policy in your Employee Handbook, and, if relevant, identify the specific positions for which you will sponsor employment visas.
Either way, make it part of your company policy to be covered under discrimination laws.
Technical Assistance Letter, Sept. 27, 2010 - OSC - This letter covers job postings for sponsorship