A teen-ager was in custody in San Francisco on suspicion of wounding a security guard and killing the manager of a hardware store. Police said the 17-year-old boy, whose name was withheld, entered the Builders Supply Center in the Mission District shortly after closing time and was stopped by the guard. The youth drew a small-calber handgun and shot guard Clarence Ball, 59, of San Francisco in the abdomen, police said. Investigators said the youth then forced manager Robert Burrell, 66, of Hillsborough into a back room and shot him fatally in the head with Ball's .357 magnum. A passer-by notified police, who said they found the suspect trying to escape through a back door. Ball was hospitalized in serious condition.