I have a different model blower, but I am able to use zip-ties to control the throttle on my blower. Just have to tighten the zip-tie to where you want the rpm of the blower. I have 3 zip-ties around the handle of my blower and I just slide the zip-tie over the throttle trigger to keep it at the rpm that I want. I use just enough throttle to move the material over the riffles so I will not blow the gold out of the machine.
There is also a thing called a "Cable Clamp" (you can search for it on amazon and places) that should also work to hold the throttle, it is an adjustable heavy plastic clamp.