Word, Idiom, or other Fixed Expression for "Stopping someone from doing something"

In my office I had stopped one of my colleagues from working on a project.
And I had to inform it to someone.

And using sentence

I had stopped him from working on it.

shows a bit of less respect for the person who was asked not to work on something.

I searched for it and found words "Restrain" & "Refrain".

Which of these(or any other word) would be best to use here?

And is there anything other than this (like idioms or phrases) to use in these situations? and sound a bit more formal and respectful?

Thanks.