Heh heh. Patience, patience.
If it is the guy who owns the truck, and pays for the service........not much you can do other than try to establish some basic rules of how you operate.
You can tell the owner that your insurance policy does not allow for persons other than you (and perhaps a helper who is on YOUR payroll) to be in the work area while you are actually performing a service task.
If it is a bunch of the owners paid employees that want to show up and hang out, the owner might want them to get back to work as they are loitering on his dime.
If it is any other kind of persons hanging out, you can tell them yourself to vamoose.
Aside from some obvious dangers of potentially spraying onlookers with fluid, or something falling/tipping and hitting them, you have to understand that some people (especially the bill payer) consider that the entire property belongs to them and that they have an inherent right to watch what you are doing to their property while on their property.
Some may be just overly curious, others may not trust that you are spending your time efficiently (over billing for wasted time, as if they could actually be the judge of that).
The concept of "sidewalk supervisors" is nothing new.
If I were in your position I might say "If you want to watch me, do it from over there (point to wherever) and don't interrupt me with conversation and the job will go more efficiently. I don't want to have to worry about you getting injured because you're too close to the work I'm doing." Or something to that effect.