Attenuation of HC-SR04 ultrasonic sensor

Hello!

I am building a robot that detects and collects objects in a short distance from it (no more than 2 meters). I am using HC-SR04 ultrasonic sensor (datasheet) to locate items. The robot will be presented in front of an audience and there is a problem. If people will stand too close they will be treated as objects. I thought about the fence, which would limit the area in which the robot moves. However, the fence also must be invisible to the robot so that the robot could see only the items that it want to collect.

Is there any material which attenuate ultrasonic and can be used for building such the fence?

Thanks in advance.