How do I disable the middle mouse button click?

Most mice are easily disassembled and fixed; I've done so more than once.

You will need a small screwdriver (e.g. those in eyeglass repair kits) and fine-tipped tweezers. Find an assembly diagram for the particular model, or just look for a screw or two on the bottom, sometimes hidden by a label or underneath PTFE pads (used to make the mouse glide better).

Remove the screw(s) and gently, slowly pry top from bottom, being careful not to drop loose parts inside -- note where the parts go, such as between button and microswitch.

If the middle mouse switch has an open spring, carefully bend the spring so it presses up against the wheel a bit harder. Alternatively, push a bit of springy closed-cell plastic foam between the switch and wheel so as to increase the force needed to depress the switch.

Reassemble and test.

This is far more satisfactory than disabling a useful switch.

However, you can disable the switch in hardware, if you must: use the tweezers to repeatedly bend and finally break off a wire going to the wheel switch. This "fix" will work with all computers and all OS's.

Install this softrware https://www.highrez.co.uk/downloads/xmousebuttoncontrol.htm and u have option do disable it or add new function to it.

Download and install Microsoft Mouse and Keyboard Center. From this application you can easily assign new tasks to mouse buttons including middle button or completely disable it.

Use screwdriver to open the mouse, then use combination pliers / tongs to remove upper part of the switcher that is below the wheel. (Of course there is some risk you damage something while doing so, but if do it carefuly chances are minimum)

You can then remove metalic part that is inside the switcher that way even if you click, the electrical circuit will not get connected which means there will be no electrical signal for the computer to do the middle click)