The free plugin does not offer any facility to disable the prompt. This is by design for the sake of security.
We would suggest using our commercial plugin, one of whose features is the absence of the "Select input device" prompt. Basically, when using our commercial plugins, a list of domains are provided as part of the configuration for your custom plugin and added to the whitelist. The plugin will then solely work only on those domains and all getUserMedia requests made on whitelisted websites will automatically be approved, without the user needing to validate them, resulting in no popups where users are prompted to grant permission to access the mic/camera.
*Please note that the Free Plugin has been discontinued.