Who can give you a reference?

Yes, within a few limits, someone can give you a bad reference. Generally, you select three or four people who you know will give you good references. Always talk to your references before using their names! Tell them what kind of position you are looking at. Ask if they will be able to give you an Excellent reference; sometimes we just don't know.

Generally it is fair game to call your current or prior boss. If your current boss will give you a terrible reference then you need a way to explain that to the hiring company. 'Please don't call my current employer because they will be very angry that I am leaving.' Your current employer has an obligation to be honest, but an oddity in America is that many current and past employers will only confirm the dates you worked there and the title of your position, also possibly verify your last salary. If your last boss is going to tell people he thinks you are leaving just in time to avoid being fired for (you name it) then you are going to have a hard time getting a job in this market.

People who intentionally give you a bad reference, which they know to be false, risk being sued for slander or libel, and possibly worse.