This is for a book I'm writing. How to restrain someone, either bodily or with some sort of implement (i.e. rope or something like that)?

Not a simple answer, as there are numerous ways to do so using the body, rope, tape...

Why is the person being restrained? Who is restraining them? How long do they have to keep them restrained?

If you can get someone on the ground, face down, there is a very good technique called a leg crossover hold. Take the ankle of one side, and place it behind the knee of the other side, then bend the leg up and over the ankle. The restrainer can then lean slightly against the leg, and the person on the ground will scream in pain. Lessen the pressure, the pain lessens. If you lean hard enough, you can pull the knee completely apart. This method also can allow you to keep someone under control for a fairly long period of time.

A simple hog tie is also very effective, both hands and one leg bound behind the back. You can do this with rope, duct tape, even a belt. Putting both legs in the tie can be fatal though, if you leave them face down.

There are just too many options to list...

Build a yuuuuuuuuge wall around him, like we're gonna do to Trumpf after the election.