KI has already stated that they'll never add the ability to remove a companion from your crew. If you want to avoid having "weak" companions, then I would suggest training them so they'll be stronger like your other companions.
Nope, all you can do is move them as far to the right as possible. This reduces their chance of appearing, but you can never get rid of them altogether.
You can also leave them defeated (instead of reviving them at Life Fountains), so they never show up in battle. However, Willowdreamer is correct, KI has indicated permanently removing companions is not going to be an option right now. The storyline (and narration) depends upon certain companions being in your crew.