How to remove the "cancel activity" button

Hi, community.

When you click an activity on the calendar page, you get this mini page where you can complete the activity or cancel it. I need to remove the "cancel activity" button highlighted here in red:

I figure I can use the remove operation like in this example:

diff: 
/**SCHEMA_DIFF*/[ 
{ "operation": "remove", "name": "CopyButton" }, 
{ "operation": "remove", "name": "DeleteButton" }, 
{ "operation": "remove", "name": "OpenButton" } }

But I can't find  the schema I have to modify.... Does anyone know which one it is?

Thanks!