Hello friends!
This block error shows mainly because of number of wrong payment attempts. The GMAT only accepts credit card. If you are trying to use any other card, it will not work and will probably block your account.
If your account is already block now, then the only solution is to call them on your regional office number which is available on the gmat site.
You will have to confirm your details to them, only then they will remove the block. Call them as soon as you can.