Employment Law Questions? Ask an Employment Lawyer.
A former employer is only allowed to make truthful and factual statements about former employees. So if an employee was fired for stealing, for example, there would be nothing that employee could do legally to prevent the former employer from stating this to potential employers.
Employers cannot make value judgments, like "employee was lazy" or things that are not true.
Most employers avoid as much as possible for fear of a lawsuit, but it doesn't sound like your former employer is doing that. So the first and most obvious question is "How do you know they are saying bad things?" and then the next question is "Are they saying things that are not true?"
If they are saying things that are not true, then your remedy would be to sue the former employer for defamation of character. Usually, a cease and desist letter from a lawyer will get the job done. You can hire a lawyer to write you a letter threatening the employer at a pretty small cost. This is often effective even if the employer is acting lawfully.
Thanks for your help.