Finally, after getting familiar with the LUA scripts I found out how to do it.
To enable/disable the visualization of the emission radius you have to use the API function simSetBooleanParameter() proposed in the other answer. In particular in the LUA script of the transceiver, after the initialization you'll need the following instruction:
simSetBooleanParameter(sim_boolparam_force_show_wireless_emission,false)
Or if you don't ever want to turn on the visualization again just delete all the occurences simSetBool related to the force_show_wireless_emission parameter in the script.