Sounds like your system is "water logged", i.e., it has too much water in it. You ready need that sight glass working. It may be that glass is actually full of water. I would completely drain the system (be sure to turn the boiler off first). Remove the sight glass and clean it out. Make sure valves at the top and bottom of the glass ar open and clear. Slowly refill the system and watch the glass. There should be a low water cutoff on the boiler. Often these have an indicator light. Watch for that go off. When it does, you should see some water at the bottom of the glass. The usual rule of thumb is to fill the boiler 2/3 to 3/4 up the sight glass.
If you still have banging and water spurting after this, make sure the vents on the main line are working. Next try replacing the vents at the radiator, they may have failed. If you still are having problems, check the pitch of the radiator. It should pitch slightly, about 1/8 in per foot, toward the steam valve. If none of these things fix your problem, you should find a heating contractor with experience in steam heating. The system was probably modified incorrectly and you need someone with experience to determine the problem.
BTW, instead of simply "topping off" the boiler when the water gets low, you should drain off a gallon or 2 of water every couple of weeks and then add water. This takes some sedimentment out of the boiler and helps keep the mineral content of the water down.