It's rude to not inform someone that they put on their pants the wrong way.
Anyone is more than welcome to voice their opinions to those that they deem as wrong as long as they keep decency. What's wrong with pm'ing someone and informing them that many players on SL frown upon this or that? Furthermore, nothing wrong with pming someone with a list of well established arguments on why you think what they are doing is wrong. Hell, it might actually be something they did not see and they will be greatful for the help.
I'll be quoting the wikia on zone-fighting again. ^_^ Keep in mind that such is placed even in the wikia of an rp site that does not regulate rp. You're brilliant if you manage to mess up even on such a site.
"Note: Using demons, curse seals of heaven, sharingan, etc that you do not have ingame, will displease many people in the zones. The best suggestion: use what you think is fair for our level."
Yeah, people can't simply accept some random acad claiming grand master level off the bat while they wasted hundreds of hour just for that particular claim.
Same for demons, some are resembling cs1 and cs2 greatly, others even go to surpass it by far, all w/o lacking it, if custom demons are to be at all acceptable they should be limited by seal1 and 2. Both becoming and keeping jinchuriki is a big hassle. Others shouldn't just go and do it w/o such just by coming up with some random custom demon Bull. Basically, it's not quite right for those that have done nothing to earn it but instead their only merit being refusing to rp as shinobi, as most do and demand, and by doing such get a strong advantage over the players that consent to SL standards.
What else? Oh yeah, crossovers are fine and dandy, but keep in mind this is the Narutoverse, so Narutoize your stuff, use chakra, not magic/Reiatsu or w/e, use jutsu not spells. People relate to the Naruto verse to chack a tech for godmodding. The shinobi and bleach were split, don't just add a inuyasha legends and stuff.
Mind their own business? I think not. There is always the chance to encounter said role players in rp/battle and what not, this is merely a precaution
against and argument erupting in the midst of battle.
So what do I do when people come ask me to see if that guy modded, explain why, etc, or just elaborate it to them cause I explain it better than the one requesting? "Sorry, bra, Kay says to stop helping others out in regards to proper rp. No can do. ;-;"