Keyboards are just apps running on the device, so no, you can't modify the keyboard (at least not legally in a production app, you might be able to find some crafty way to do it if you don't plan on putting the app in the playstore).
I'm not really sure what you're trying to accomplish, but some other options you might look into are:
- You can write a new class which extends EditText and use it in place of native EditTexts. If you do this, you can override the behavior of the cut, copy, and paste actions, so whether they come from the keyboard or touch screen you could just make them do nothing, kick up a toast, etc. See this question: Android intercept paste\copy\cut on editText
- You can override the action mode callback of the native EditText as well, so depending on your target API level this might be a slightly easier way of achieving the result from number 1. See this question: How to disable copy/paste from/to EditText
Hope that helps somewhat!