Vendor Blocks in SRM

Hi Guru's.

Please can someone explain what system changes we need to make so that a errror message is displayed during Cart creation to tell user that a Vendor is Blocked. The message only appears on the PO screen and it is our requirement to make the user aware during the Cart creation process.

Our vendor blocks are maintained in R/3 and sync'ed across to SRM 5.0

Please advise.

Thank you.

Viju