It’s kinda boring if u r the Pc wizard and the DM is doing that to you every round.You joke, but it would be awful nice if you could at least cover the mouth of a caster, maybe after grappling and shoving prone. Either that or hold their hands flat (so no somatic) - but not both at the same time. I guess you can apply manacles, although, there are no rules for applying them, and of course while you are busy doing that your target can simply misty step (or dimension door! or thunder step!) to safety.