How do I disassemble this

[Chris09]

Just an idea. Drill a hole through a block of wood a bit tighter than the diameter of the inside threads, cut it in half through the circle along the wood grain and squash the two haves with the threaded portion together in a vise, might spread the load around the diameter a bit, then if you have a small wood or C clamp squeeze the two block haves in the opposite direction a bit so the wood doesn't split.

 

[Ron]

Get a nut the thread size of the showing thread. Cut it in half (across the points) so the cut is parallel to the 2 opposite flats. Put the 2 halves on the threaded stud. Put the assembly in a vise and tighten firmly. Keep in mind you may also be crushing down on the smaller allen screw. Using the above mentioned impact driver try to loosen the allen bolt. Apply some heat (heat gun/hair dryer).

Or

If the split nut doesn't hold, use the split nut idea and use red Loctite to secure the 2 halves and threaded stud. Let set up in tightened vise for 24 hours. Try to break it loose as above. It should be fairly easy to remove the nut's 2 halves.

 

[Andrew Shadow]

Are you attempting to keep the old bar-end weight for future use?
If not, it is hard to tell by the picture but, the large hex-head fastener looks like it might be press-fit in to the exterior round part. If so, hammer it out of the round part so that the hex-head can be securely held in a vise. Alternately, just cut the exterior round part off. Hopefully it is then a matter of a combination of penetrating fluid, heat and force.