Repeat after me:
"I'm sorry, I'm not allowed to say - commercial confidentiality."
"I'm sorry, I'm not allowed to say - it's a security issue."
"I'm sorry, I'm not allowed to say - they'll hunt me down like a dog and dump my body in a wheely bin."
If they still keep digging, hint even more obviously at Security Service or Russian Mafia involvement.
PLEASE NOTE: such techniques should not be attempted by those inexperienced in confusing stupid people who make their living talking on telephones.