Why can't Mr Ford just prohibit Tibeats from killing Platt in 12 Years a Slave? He does have the power to do that since Tibeats works for Mr Ford. And Platt was officially Mr Ford's property. Doesn't that mean it's illegal to kill/destroy someone's property?
While Ford was pretty much on the cusp of being a complete abolitionist, he still had a reputation to uphold, even if he didn't want to. He clearly didn't like the slave trade, but as his character is designed to show that not even abolitionists could escape it, especially if they lived and operated in the south. Every other plantation owner around Ford was using slave labor to get their work done, and if Ford attempted to do so honestly, he would likely be unable to compete, and further would be viewed as a sympathizer, something strongly frowned upon in the south during that time.
This is evidenced in the dialog between Ford and Solomon while Solomon lie in the hallway of Ford's home:
I am trying to save your life! And... I have a debt to be mindful of. That, now, is to Edwin Epps. He is a hard man. Prides himself on being a "nigger breaker." But truthfully I could find no others who would have you. You've made a reputation of yourself. Whatever your circumstances, you are an exceptional nigger, Platt. I fear no good will come of it.
Word has already spread of what Solomon has done to Tibeats, and if Ford defends him he runs the risk of being seen as a sympathizer, and even worse as a slave owner who allows his slaves to rebel against their white masters and get away with it. That would literally ruin Ford's business, and quite possibly cause harm to come to him and his family.
By transferring Solomon to Epps, he was able to save Solomon by keeping him out of Tibeats' reach, while also being able to say he sent him to, as he describes, a "nigger breaker". This is compounded by the fact he believes Solomon would further attempt to retaliate against any harsh treatment by Tibeats, which would only add to the reputation problems Ford was faced with.
The period of slave trade in the US was a very hard time, with lots of social and political pitfalls that could ruin you were you not careful enough. Decades of effort were spent in its abolition, and Ford was simply a man unable to act due to the circumstances of his own life and business based on his location.
Mr. Ford was afraid that Tibeats wouldn't be rational, and attempt to kill Platt without thinking (or caring) about the consequences. He probably thought it prudent to remove the temptation away from Tibeats completely by placing a guard, rather than trust to Tibeats' common sense.