How to stop the popup message of "out of definition" from require SEPM group?

Check this article

Endpoint Protection client warning: "Old Virus Definition File"

Login the SEPM Console

Go to Client

Select Group then Policies Tab.

Edit the Virus and Spyware Protection Policy(if policy is shared then create it non shared policy)

Go to Miscellaneous from Advance Tab

Go to Notifications under Miscellaneous 

Uncheck the "Display a notification message on the client computer".

Click Ok.