It depends on what "locking up" means. If we are talking about US-style prisons then letting (possibly innocent and for that matter even guilty) people languish there _is_ cruel and unusual punishment.

However, there are better "prisons" (see e.g. what Norway does) and it is possible to let (dangerous) people live lives that are dignified, fulfilling and productive (when they are able to work) under the constraints of captivity.