Reducing trackpad sensitivity

Hello, I need help setting up a MacAir for someone who uses a finger and the thumb to hit the trackpad. (Both move together.)The pad reacts by selecting text - a line or even a whole paragraph. How do I get the system to see this as just a one finger gesture? They just want to insert a cursor and then type, not select whole blocks of type or lines on a spreadsheet. Is there a sensitivity adjustment I can use?