I would agree with the other answerers in saying that as long as it is truthful, they can say what they please. The reference is being asked to provide their opinion on whether or not you would be a good choice for the prospective employer. They have every right to say no if they want to. Sucks, but true.

Not unless they are saying something that isn’t true. And by the way, good employees don’t typically get written up twice in 6 months.

If you sign a release statement that releases former employers from liability for information about you then they are allowed to disclose any truthful employment related information about you. They can not make up stuff about you, but they can give out information about you and your work for the company. Some may tell you that employers can only give out name, positions held, dates of employment, but that’s not true. Most employers will not give out a lot of information, but since you sign a statement to run a background check on you and release them from liability then they are allowed to do so. You could only go after them if they are saying things that are not true about you.