I've added this to may pally tanking macro:
/script UIErrorsFrame:Hide();
/script UIErrorsFrame:Clear();
/script UIErrorsFrame:Show();

It has removed the red text in the middle of the screen, which says "spell is not ready" or "you cannot attack that target".

However, my general window is spammed with that information. I don't see an option in the filters to turn it off. Is there a way to turn this off or disable the message?