If that someone is a police officer, there are strict limitations on what the officer can do on your property. The normal requirement is that to enter a person's property police must have a warrant, and if they do not have a warrant, they must believe that there is some specific justification to enter your property. If that someone is not a police officer, they may be trespassing on your property.
***This is NOT legal advice. For legal advice tailored to your situation, consult an attorney.***