At what point can we dismiss a person's argument due to lack of intelligence evidenced by poor grammar and spelling? Of course, let's assume they are native English speakers so we wouldn't apply this to ESL people.

For me, if you can't differentiate they're, their, there, OR properly use then/than, OR say "could of", "should of", or "would of", then I will apply the "language brainlet" rule and dismiss your argument.