While the mode setup we have is not how I'd have done things, I don't have a huge problem with it, I completely accept that it's a part of the game that's not going to change, and it's not at all what I was referring to.
The block function is easily abused precisely because it does much more than just keeping people you don't want to play with away from you. It's an arbitrary exclusion filter that applies to each and every instance you find yourself in. People can, unwittingly or intentionally, interfere with other players instancing with each other, not just with the blocker. If someone does this intentionally, and not to keep away players they don't want to play with, but to prevent players that want to encounter, or are not opposed to encountering, each other, that is clearly abuse. This is extremely hard to prove without an admission, but I'm convinced I've seen it on several occasions, and anyone can demonstrate how the function can be used in this manner.
The menu log/mode switching functionality can also be abused, offensively, to disrupt the legitimate gameplay of others. I just PMed you with links to a pair of videos where a hostile CMDR, who was fighting for the opposite side in a war repeatedly logged in and out of Open specifically to stall our victory in a CZ. I used to see this sort of thing all the time, essentially every CG.
Indeed, most of the cases of 'griefing' or harassment I see in Open have little to do with PvP combat, they either involve out-of-character slander via chat, or they have people picking fights and then using the mode/instancing tools the game provides to waste people's time or break people that want to play together apart. Ships and guns are not the primary tools of griefers and rarely have been...they're inefficient and unreliable for that purpose.