I think the line that separates a "clean" and "dirty" driver is the "intention". Accidents DO happens, but If someone is grossly over speed into your inside to "over take" and rams into you, then its no accident. If someone repeatedly ram into you from outside while you are on the inside, side by side, then its no accident.
An unspoken rule in PC sim racing is if you hit someone and know you are at fault, you will stop and let the guy you hit on his way back in front of you. Close racing can ONLY happen when there is a trust exist between drivers, where each of them knows they will NOT give a position up willingly yet provide just enough room for the other guy to survive.