Come in two flavors; true ore thieves and PvP cowards.
I don't hate the first flavor. They take advantage of the fact that miners employ an accepted exploit (jet cans were never intended by the devs to be a mining tool) that leaves open the opportunity for theft, without consequences to the thief and only small, avoidable consequences for the victim, as long as the thievery is executed correctly. These guys are like shoplifters. What they do is wrong. It's a pain in the ass. But they will never cost me a ship. These guys work in teams of 2 or 3, with a flipper in a fast, small ship (shuttle), a hauler with a tractor and an optional scout. The shuttle drops a can right next to yours, the hauler targets the can, flipper starts a warp out to a station and (while aligning) moves the contents from your can to his. As soon as he hits warp, the hauler turns on tractor beam, pulls the can & picks up the contents. Flipper docks & waits out the timer. Not flagged (he didn't take from your can!), Hauler docks with your ore. Rinse & repeat. Done correctly, you never have a chance to engage these guys.
I hate the second flavor. These are school yard bullies. 14 year olds who hang out on the elementary school play ground, stealing lunch money from 8 year olds. They want a fight, but only against the weak. They flip cans in combat ships and hope that they will be underestimated by a new barge pilot. They want to blow you up. If they get some salvage or loot from it, fine, but that's not the point. If you choose to ignore them, they smack in local or open a convo to tell you how lame you are. Again, hoping that you will engage, so that they can blow you up. If you have backup, they hope to use aggro timers to allow them to isolate weaker teammates to kill after the rest of the gang's aggro timers expire. These PvP wannabe's could go to low sec or 0.0 and get all the pretty lights they want. The problem with that is they might actually have to face a fair fight. So, they pretend to be tough guys & pick on nOOb miners.
It's easy to avoid both types and to keep your ship facing the second.
1. Don't mine into a jet can. For new miners, use a GSC, put a password on it and haul it when it gets full.
2. Mine to a jet can, but have a hauler standing by to move the ore to their hold. Put a single round of ammo or a bookmark in the can ("popper stopper") and transfer the ore as soon as it's dumped. If a can flipper flips 1 round of ammo, laugh at him in local. Partner with another member & swap off hauling duties. Split the take 50/50.
3. Ignore them. Follow Silentbrick's procedures for reporting a can flipper in Corp chat and then do not engage. Remember, they are flagged as thieves to the whole corp, but they can't shoot at you unless & until you shoot at them. YOU have control of the engagement, not them.