I'm going to prevent my brother in law from getting a job.
Why would you do this to him?
I'm going to give him a bad reference.
It's inappropriate to try to stop your brother in law from getting a job. He asked you to give him a reference so you should give him a good one, because that's the right thing to do.
I have unlimited resources, get him fired and ruin his life.
You should let him get a job and be supportive. How would you feel if someone tried to sabotage your job prospects? People need to work to be able to support themselves.