EDIT: oops, I misunderstood the question.
You don't want to 'disable' Caps Lock because how exactly would you communicate to users that this mode is unachievable? Some users may not know how to use Shift to temporarily enforce uppercase, and may not be able to type their password anymore.
Besides, it seems unintuitive that a functionality of your keyboard becomes unavailable when using an input field on a website on a specific application. How would you go about programmatically disabling Caps Lock on the user's system, so that the keyboard's mode feedback is properly aligned? It would pose serious security issues if websites were able to do such things with JS code.
Old answer (offtopic)
Uppercase letters are distinct from lowercase ones in passwords because of a thing called password entropy, aka. the number of significant bits of information in a password. Having a larger alphabet of symbols to draw from increases the entropy of the passwords*.
By removing uppercase symbols, you would thus lower the security of your passwords. Because the caps lock switch is a modemode that is maintained on its own without continuous user action, it is necessary to display feedback in situations where the user must be aware the mode is enabled in order to fulfil their task successfully.
(* this is an overly simplistic view, many people in security research miscalculate password entropies by not taking into account the relative probabilities of some symbols appearing at some positions of some passwords. Still, the general concept is that more characters to choose from lead to a potentially higher overall entropy of passwords)