Harmonics are trivial to filter - they're at multiples of the fundamental frequency, so 100MHz, 150MHz, and so on. You can just stick a lowpass filter on the output and it'll block all the harmonics because they're so far away from the fundamental. Unfortunately, the waterfall appears to show strong spurs every 30 Hz, and those would be ridiculously difficult to filter out. I think the normal solution is to design your transmitter so that it doesn't do that.
It looks like they're spaced at 30KHz, which seems to me to be a very tight grouping for any kind of transmission like that (and presumably if that band was that crowded to begin with, the author would have picked a different one).
