that actually isn't that hard if you are okay with astral projection, or at least a good ability to invision.
I have two methods there very similar but one gives you a physical prison to put them in.
physical prison method:
obtain a quartz crystal and then meditate within a protected circle (this isn't exactly the most positive of magick so I would have some good protection/defenses up) you should be in the center of your circle, now either astral project to an area that you have created to fit this situation or invision it (requires a good deal more concentration)see everything as if you are looking through a veil or standing on the parameters of the area, not inside it. the area should resemble a prison of energy if you will, you should probably throw some binding sigils or runes in there somewhere, but also if the person is someone you don't want to necessarily suffer, throw in some protection from outside forces so nothing nasty stumbles upon the target. you can either see yourself paint these or put them up astrally in the air. See all of the restrictions and realize there is no way for your target to get out.
Personally I invision two circles, i astrally paint them on a ground. leaving a space for myself to exit, I lace the outermost circle with binding, restricting, and holding runes, then on the inside is drawn a pentagram through the whole circle to insure their protection. near my exit I will draw the targets sigil or place something that is related to them (ie, a picture article of clothing or a sigil I have drawn to represent them), in the center I place the quartz. now stepping outside the circle, and closing it, I draw another circle, (or the tirangle of solomon but not everyone uses that form of magick so its easier to describe with another circle)
Now in my protective circle, I meditate on the target, and call them to the quartz crystal. When I can see them there, I attach shackles to the arms and have them fall to their knees before me. now completely immobile and imprisoned with no escape possible invision the entire circle you have just created (containing your target) being trapped in the quartz only to be let loose with its destruction or a despelling of some sort.
Same as method one but without quartz, simply state that they are imprisoned. then close your circle and leave the astral place.
with both of these methods I suggest visiting your prisoner at least once a week to insure they are still there and well off, as good as they could be under such conditions anyway!
I have only kept a prisoner for about a month, I never thought of the concept but it was just brought to my attention, Should they be fed? like energy or something, everything needs some form of sustenance to survive....hmm...