Last night I couldn't enjoy a Quna concert without certain players posting nude symbol art in the lobby! I don't know why Sega allows them to do such. I ended up leaving as it ruined the whole PSO2 experience for me!
From the online manual:
"Report Function for Symbol Art
Navigate to the Main Menu > Symbol Art > History. You can select the symbol art you would like to report by clicking on the art and selecting “Block.” Once you do this, you will not see the symbol art anymore."
There is also an option to just disable all symbol art in the Options.