Dear (family, friends, neighbors, etc.)
It is with deep (sadness, regret, reluctance, displeasure, joy, frustration, etc.) to inform you that I will not be able to perform that background check on your (mother's boyfriend, daughter's boyfriend, roommate, tentative employee, neighbor, etc.) that you asked if I could conduct multiple times.
For future reference, I will not be able to perform background checks on your (mother's boyfriend, daughter's boyfriend, roommate, tentative employee, neighbor, etc.) at any time in the future due to the user agreement that I signed upon becoming a sworn officer. I can't do this ("just this once", or "without anyone knowing", or "in 30 seconds or less", or "because I'm your mother").
Performing an unauthorized background check would be unethical and violate the oath that I have taken to protect and uphold the constitution of the United States. It also is a violation of the civil rights of the individual you would have me check. Not only would I hate myself for breaking the law, but I would risk unemployment and decertification. Not to mention the humiliation and shame heaped upon me by my fellow law enforcement officials. I would also lose the trust of the general public. You wouldn't want that to happen to me, would you?
Just thought you should know,
Your (friend, son, brother, sister, spouse, neighbor, girlfriend, boyfriend, roommate, etc.)
Officer (fill in the blank)