Destruction catalyst, nova catalyst, Gem Armor, Morning Star, Collectors for sure.
I may love EE but those can effectively end a hardcore PvP server. Once I used a morning star to make someone's base go away in one hit, felt really cheap afterward. The catalysts can do same thing but slower, Collectors = free stuff, Gem = Invincible. Consider removing the condenser since a half decent darkroom and put out endless diamonds, but can also ruin a lot of other complex builds, which I have seen some big ones even on a PvP server.
Some people feel IC's Nano armor and EE's redmatter armor is OP as well. Basically just go by ear and see what people are abusing the most and consider those for banhammer. That's all that comes to mind right now but the ban lists quickly grow so just keep an eye on number of items banned or it gets out of control.