You can use the List Action Remover to both remove Custom Actions from the list ribbon and also to add your own script to web pages within the site.
First, click the "Go" button to get to the settings interface:
From here you can disable the buttons for both Custom Actions and custom built script buttons from the selected List. Use the "X" under Actions to remove the button from the item's and list's ribbon interface.
If you need to add in a script to load on every page, you can use the "Add Web Script Link" section to add in your own custom script.
