How to deal with worn-out threads on binding post?

My speakers have Edison binding posts. The copper threads on the binding post have worn badly over the years, so that the nuts do not screw on tightly. I would prefer not to replace the binding posts, in part because the speakers weigh 250 pounds apiece (not easy moving/transporting them), and in part because the speakers are one-off and the designer is deceased. I use speaker cables with spades, and banana plugs are not an option.

Any suggestions as to how to get the nuts to hold the spades tight against the binding post?

You can use a die to rethread the post to a smaller diameter.

You can find dies and Nickel-Copper nuts at McMaster-Carr.
Couple of suggestions:

Put some flat washers on first to move the tightening area further back, if there's room. Perhaps the threads are in better condition.

Try a nylock nut.

A piece of small diameter solder through the nut before you thread it on the post may help to fill the gap. You can even try dental floss or something similar.

My philosophy is to always try the easiest fix first.
When I owned Edison Price binding posts, I had the same issue. Threads are not worn out, the split barrel has become compressed over time by the nut.

Don't laugh, but you can cut a very short piece of wooden dowel rod and press into the center (between the split halves) and return it to it's original shape. Leave the wood in full time, it helps dampen the posts as well as stopping further compression.

The only caution, select the right diameter dowel rod so you don't crack and break the Edison price binding posts. I've also had great results putting Caig Pro Gold on the dowel rod and connector. Everything stays clean, conductive and free from corrosion.

Let us know if successful for you.
AlbertPorter: That looks like exactly what has happened. In fact, there has been so much compression that the two halves now look like an oblong

Thank you so much for the advice! I will be careful in choosing the right diameter dowel, and will let you know how it works out.
It worked!!! I inserted an awl into the space between the two split halves of the barrel, and gently worked it back and forth to move the two pieces more or less back into alignment. The nuts now screw on tightly, and the speaker cable spades are now held securely in place. The next step is to find an appropriately sized insert to keep the two halves in position. (A friend suggested using a small diameter bronze rod.)

Thank you again! It's amazing what a tight cable/speaker connection will do for the sound!
I’m, glad my comments helped.

The Bronze rod your friend suggests is not the same conductivity as the original Edison Price connector and adds additional material the signal must pass through. I would still advise an appropriate diameter wooden dowel to keep the split halves from compressing. The wood rod does not alter the original conductivity and unique design of the Edison Price.

Too, the wood dowel has some give as you tighter the nut. This helps with resonance and provide a micro amount of "spring" or friendly resistance to the connection.

I know I'm being picky, but the effort is low and the cost of the dowel rod is less than the gas to drive to the hardware store. Try both ways if you're adventurous, but please at least try the wood. It was a very nice and basically free improvement when I was using Edison Price.