Jeremy,
Let me interject here.
If ALL of your lower vents are closed, and your temperature is still too high - chances are that something else is wrong.
In order for the kind of combustions to take place, that allows high temps to occur, the fire needs fuel, oxygen/air, and exhaust. The WSM draws-in fresh air from below, and exhausts warm air and smoke from the top. Under normal circumstances, the WSM responds well to a reduction in any one of the three. If you close the bottom vents, it really should not be able to draw enough air to sustain a significant heat source, and should "ckoke-down".
My guesses would be one or both of the following:
1 - LEAKING AIR (SOMEWHERE?)
Did you correctly assemble your WSM? Some WSMs arrive somewhat "out-of-round" (measure the middle section across at the top & bottom in different directions, and note if there are significant differences). Most are out-of-round a bit, some more than others. I rotate the mid-section when I seat it on the lower (fire bowl), and find the "sweet spot" - where it fits together easily, and seats in all the way. If it's not properly fitting together, it could be drawing air through the gaps where the different "ovaled" sections meet.
2. SOMETHING WITH THE FIRE
Maybe you're waiting too long to assemble the smoker after you get the coals going? Or using too much fuel for a small load of meat. (although a proper, slow-burning fire, rarely causes "runaway" temperatures in a WSM that is set-up properly - expecially once you shut-down the lower vents).