SCTM: Assign user to execute test definition.
SCTM: Assign user to execute test definition. After we have assigned tester 1 to test definition 1, then the test definition 1 should block to other tester.
By using Manual Execution Planning and Test Cycles resolves this issue. When you assign a test to a user then this test in available to that user only for execution within the test cycle.
Please provide update as to if you are using test cycles. As this will perform as you are expecting.