BDL Controller described an unauthorized runway entry event by an air carrier taxiing past an aircraft holding short of the runway; the reporter noting clearer LOA procedures are necessary.

Narrative

A B737 was instructed to taxi up to and hold short of Runway 24 at BDL. The pilot read back the hold short clearly. This was confirmed by a tape review. The aircraft then taxied past the hold short line and on to the runway. I asked him what they were doing and reminded them they were told to hold short. There was no response. A Dash 8 was on short final and was sent around. Prior to the event there had been a confusing situation involving a Cessna parked in the 24 hold pad and the B737 aircraft in question. This confusion may have led to the lapse in concentration that led to the pilot deviation. Recommendation; the pilot in this event may have been confused by the prior instance when a Cessna was parked in the 24 hold pad. Both aircraft believed that they were number one in sequence despite the fact that I had originally told the Cessna that he would be number two. There is an LOA at BDL that does not allow aircraft to pass each other if one is parked in the hold pad and this LOA did nothing to remedy this situation. Aircraft routinely pull into the hold pad without advising ATC and the Local Controller has to divert his attention away from the runway in order to remedy the sequence issues caused by aircraft pulling over. A better LOA is required ASAP in order to minimize confusion.