FF68 : User Exit (to restrict customer)

Dear SDNs,

Basically I am SD Consultant.

My client has requirement :

If Customer is Blocked (XD02 > Extras > Blocking Data), system should not allow to do FF68 for that Customer Code.

I have set the blocking data in XD02 as below:

******

All company Codes : KNA1-SPERR

Selected Company Code : KNB1-SPERR

Order Block

- All Sales Areas : KNA1-AUFSD

- Selected Sales Area : KNVV-AUFSD

Delivery Block

- All Sales Areas : KNA1-LIFSD

- Selected Sales Area : KNVV-LIFSD

Billing Block

- All Sales Areas : KNA1-FAKSD

- Selected Sales Area : KNVV-FAKSD

******

Now in FF68 what user exit can I use?? Where I can check the above criteria and give the error message..

Or any other option is there??

Thanks in Advance..

Regards,

HP