Question
I have a Mexican fence cactus that is about 20 years old. I cut the tops several times and potted them in the same pot. It is now so root bound there is no space left to re-pot any more cuttings. Is it a good thing to leave the cactus root-bound in the same pot without planting any more cuttings? Also is there anything I can do to keep it from getting top heavy so I don't have to cut it?

Answer
Cactus like to be root bound but more importantly when's the last time you changed the soil?  If that hasn't been in a couple of years, you need to plant it in new medium and for columnar cactus rule of thumb is the pot is 1/3 the size of the plant. With a cactus of your size which sounds big the width should be so it leaves about two inches around the plants. It is a good opportunity to check the roots and make sure they are white and heathy looking. New cutting can start their own family in  another pot or give them to your friends.

The only way i know to help it from getting top heavy is to remove the arms at the top as you have been and hopefully some will sprout lower down, or you can cut off the main part and let side shoots grow.  Eventually they should hide the wound. If you do that make sure you brush the wound with  fungicide, rooting powder or charcoal dust to prevent microorganisms from getting in. The proper size pot and some staking may also help.  However it is the nature of the beast to grow upward. I'm sure you've seen how large these plants get when planted outside so it is a battle to control them in a confined space.  With a little ingenuity you can do it.  Also, be sure you don't water too much.  it will make the cactus grow faster (or kill it) but the new growth won't be as strong. And don't water in the winter.

A nice winter challange!