I frequent a public range where there can be anywhere from zero to four people on site who are there for the sole purpose of scrapping brass for about $1.00/lb locally.
**Most** of the time they are very polite and over the course of weekends, you get to know the people who are scrapping. Some are retired, unemployed, want a little extra spending money, etc. They will usually ask, "Are you keeping your brass?" It gives the shooter the ability to answer. However, if they don't ask and they are awkwardly hanging around, I clearly communicate that, "I am keeping my (enter calibers here) brass." If a person is at all reasonable, this should be enough. They usually don't know what your're shooting or even what caliber it is. They just see it as pennies and nickels on the ground.
There have been times when someone is just unreasonable - ie they are scrapping......
too close to your bench while you are shooting
behind you as you are shooting
around your bench during cease fire, while you are changing targets.
Action items:
Let's say that you have brass that you need to keep. I have a sharpie with me. I will just mark the case heads (or previously have marked them). That way if you see someone picking it up, it's as easy as, "Did you pick up my (caliber) brass? Mine are ___ headstamp and marked."
If someone is so brazen that they don't care, I would suggest one of a few things:
The first option is just give them the brass. If it's not high dollar stuff, just let them have it.
The second is to shoot something they don't want for just a little while. It's better if you don't have all your ammo laid out like a buffet. Pull it out of your bag as you use it. If someone is after your 308 Win cases, for instance, just take a break. It's a good idea to have a second gun to shoot something steel cased. You could bring out a 22LR. You could switch to a single shot, bolt action, or revolver for a string. Scrappers don't want steel casings and it takes a determined person to camp out behind you to collect your 22LR. If you start extracting your revolver cylinder directly into your bag or bucket, there are bigger fish to fry.
The third option is to slow down your semi- auto fire. Take your time and space your shots. Scrappers want to collect behind shooters doing mag dumps. Even if you're shooting once every 30 seconds, they are waiting for just one casing. There are better opportunities hawking other benches.
I haven't done this personally, but there is a "nuclear" option for "over the shoulder brass hawks." It's called a muzzle brake. If you start shooting something loud or obnoxious, where there is a concussion or gas is puffing them in the face, you have made it undesirable for someone to be there.
Lastly, collection devices. Laundry hamper, tarps, butterfly nets, gun specific case collectors. Scrappers want opportunity, if you aren't letting it hit the ground, there is no reason for them to stand around and wait for nothing. It's just that the collection devices need to be unobtrusive. The fun of shooting is testing/training or getting better at shooting. Having too much focus on collecting every last piece of your brass or being in the perfect position for it to extract into your collection device usually means that you won't have fun shooting.