How can I clear event subscriptions in C#?

Add a method to c1 that will set 'someEvent' to null.

public class c1
{
    event EventHandler someEvent;
    public ResetSubscriptions() => someEvent = null;    
}

From within the class, you can set the (hidden) variable to null. A null reference is the canonical way of representing an empty invocation list, effectively.

From outside the class, you can't do this - events basically expose "subscribe" and "unsubscribe" and that's it.

It's worth being aware of what field-like events are actually doing - they're creating a variable and an event at the same time. Within the class, you end up referencing the variable. From outside, you reference the event.

See my article on events and delegates for more information.