A steam radiator should not be blowing water or steam out the vent. The purpose of the vent is to let the air out and keep the steam in. All the water should fall to the bottom of the radiator to return through the pipes to the boiler to be used to make steam again. You need a new vent that has a hole sized to time the amount of air vented so the radiator gets warm at the same rate as the other radiators.
There are thermally activated radiator vents available. They are not the cheapest, but the best for regulating air and the heating rate of the radiator. They are available on order from plumbing/heating suppliers locally.
The water in a steam radiator should not be high enough inside to blow out at all. See if the pressure in the boiler runs over 2 psig on the gauge; it should be below that at a maximum of 1 psig. Also check that the water level in the gauge glass is more than 28 inches below the horizontal pipe that leads to the radiators (the main).
A good site to learn more is at http://www.HeatingHelper.com