Actually an employer CAN NOT just say anything they want about an employee.
When a future employer contacts your former employer they should be directed to the Human Resource department to answer any questions about you. The HR person should only tell them the following:
1) Your start date
2) Your title
3) And if they would rehire you ( yes or no - no explanation).
If anything else is said - such as your boss telling a prospective employer that she/he wouldn't hire you back because your lazy - your work preformance was bad - whatever it is - you can sue them for deformation.
I just did it myself this very week and have a hands down case (thanks BOR
Call up your former companies HR department and ask them who you should direct reference calls to.

