Security Level definition states:
"The security level of the report. In order for a user to open a report, their report security level (defined on the Stat users table) must be greater than or equal to this value. Valid values are 0 - 99, with 0 equaling minimum security and 99 equaling maximum security".
1. Go to Stat Options | User Maintenance, highlight 1 user to restrict the report, scroll to the column RptAccess and set there 99 and Save.
2. Try to run the Access and run the report. The user should not be able to run it.
NOTE: Don't touch the security level yet in the Reports definition table. Make sure it has the default value 50.