Hello Ravi,
These condition table has priority sequence if you mention a dummy data in Condition table 1 you can stop MSDS to dispatch for specific customer and sales org combination but again please check with Regulatory group as this may be very much required to send MSDS if product is Environmental relevant.
As CB stated above you can mention dummy Email which always goes to email box which is not for customer another solution, also can develop customer exit which read the data from Customer master and determine the dispatch of MSDS.
Few Point to Consider -
If Certain Material do not require MSDS regardless of Customer than Use SAP Standard approach
If Certain Sales org of Customers do not require MSDS than use SAP Standard approach.
What is the business driver ??
Gopi Dadheech