There's no way to reduce how many blazes are spawned per cycle of the T5 spawner, however, it is possible to keep it on for X amount of time, before shutting it off and waiting for Y amount of time before restarting the cycle. Easiest way i can think to do it would be with ComputerCraft. Place a computer above the spawner, use this quick and dirty program:
while true do
while redstone.getInput("top") do
redstone.setOutput("bottom", true)
sleep(15)
redstone.setOutput("bottom", false)
sleep(15)
end
This will turn the spawner on, leave it on for 15 seconds, then turn it off for 15 seconds, before restarting the cycle, and it will continue to do so as long as it receives a redstone signal from the TOP of the computer so you can run some red alloy wire from the computer to outside the spawn room, to a lever, where you can turn it on and off as you please.
Tweak the sleep values to whatever works best for you. Maybe you want the spawner on for more than 15 seconds, maybe for less than 15 seconds, or maybe you want it to be turned off for longer periods of time before it turns on again. Of course this could be done without computercraft, with toggle latches, timers, etc... But this is much more compact, and i like it better.