Then we need signs giving plenty of warning on approach. As it happens, they've just recently installed some brand new signs on the A23 and A27 (not yet in use) which can display variable messages. All that is needed on matchdays for away fans or those "ignorant" of the restrictions is for them to be set on matchdays to state something along the lines of:
"RESTRICTED PARKING AT AMEX STADIUM. TAKE A27 (W) / A27 (E) AND FOLLOW SIGNS TO PARK & RIDE"
That should deter most from attempting it. Or it could be changed to NO PARKING AT AMEX if necessary.
If that was me, heading to an away ground, I'd ignore it, and try my luck anyway.