I have identified two basic strategies for scavenging someone else's spot in a group setting such as Zumba.
Nobody wants to be the victim of a Scavenger, so here's what you can do to protect yourself. First, you will need to practice self-denial. You need a drink? Use the bathroom? Toughen up, cupcake--those vulture-type Scavengers aren't going to take pity on your bodily needs, and neither should you. And to deter the bulldozer, you'll want to employ body armor. Goggles, a helmet, shin pads and steel-toed boots are a good start.
Lastly, don't let the Scavenger make you question your existence. You are visible, and most of us can see you.