It certainly won't hurt the amp by using only two out of the three available channels. As a safety concern though, just make sure that nothing comes in contact with the speaker binding posts. :o)
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Solid state (output stage) amps don't mind having no load, especially if no signal is input to that channel. I do remember that no load conditions were considered a problem with tube amps. To be completely safe, put a shorting plug on the input and a 10 ohm 5 watt resistor on the output.
By the way, the two remaining channels will be improved by having the power supply all to themselves.
El's suggestions are right on the money. The only thing that i would add would be to change the 10 ohm resistor to one of a higher value. By increasing the load resistance, even if the amp were to try and reproduce some type of signal i.e. RFI induced noise, the higher value load resistor would limit how much current the amp would be able to produce. You have less potential to damage the amp should something go haywire along with less potential to smoke the loading resistor at that same time. You don't need to worry about the specific value of the resistor so long as it can handle a few watts of power. If it were mine, i would use something between 30 - 100 ohms. Sean