Black-balling

My husband worked overseas for an off-shore oilfield company several years ago. He terminated employment with this company because they refused to allow him to return home when his rotation was over. Now, years later, he is trying to get another offshore job with a different company, but is having a lot of difficulty. It is our belief that his former employer may have said or done something to prevent other companies from hiring him. Is this legal? Can they legally do this to him, or does my husband have rights we are unaware of?
Just a personal note: My husband has seven years of offshore experience and is more than qualified for the positions he is applying for. He also has an excellent resume. This is why we believe his former employer may be behind the difficulty he is having. Also, we know that the companies he has applied to are hiring.

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If someone publicly says or writes anything factually untrue and it damages you, you indeed can sue for slander, libel, defamation, interference with business relations etc. -- or at least threaten to do so and give a warning.