When the game is showing the title menu, mouse clicks are not suppressed by EventArgsInput.SuppressButton().

Here's a sample mod that exhibits this behavior:
ButtonSupressor.zip

The source code is:

public class ModEntry : Mod
{
    public override void Entry(IModHelper helper)
    {
        InputEvents.ButtonPressed += this.InputEvents_ButtonPressed;
    }

    private void InputEvents_ButtonPressed(object sender, EventArgsInput e)
    {
        e.SuppressButton();
    }
}

The expected result is that you cannot click the on screen buttons to load/start a new game/etc, but you can. This is running SDV 1.3.11 and SMAPI 2.6-beta.14.

Fixed in develop for the next SMAPI 2.6 beta.