Glitching into textures can be unintentional; I don't remember any easy way to glitch into the San Fierro stadium. Besides, there is no obvious way of getting out once you are there (like, there's no missing ground textures inside so you could jump in the 'blue hell' and appear outside). So, no action will be taken, but I'll talk to him ingame about the impermissibility of intentional glitching.
I see that you were a member of the protection team, so here's an advice for future cases. Just bring a vehicle close to the textures and ask the bugged person to press 'G'. On his screen, he'll run at the wall towards your car, and in a few seconds he'll appear inside. Might be quite helpful if there are no staff members around to help.
Thanks for reporting.
