I understand author of this topic from several reasons. I was robbed in guarded cities, i tried this "roleplay" on the dark side in my dark game days too.
1) Danger in cities creates atmosphere of the game. Even in those guarded. It is Fonline, this feeling should be everywhere.
2) I agree, that guarded cities should be "more protected" than unguarded, but adding new kind of protection (preemptive or consecutive) often leads into situation, where "regular" players are punished and those bas....rds not, because they know how to violate it.
3) I can guarantee you, that suicide killers (bombers, one hex bursters,...) will always find another way how to play their roleplay. The fact that this kind of behaviour is forbiden only encourage their effort, their innovation.
4) During my personal observation, i realized that players think that guarded zones are safe zones. So they can forget about security. I.e.: players ignore situations of dead corpses of players or guards, they ignore inventory checks of mates...
5) Solutions? Leave it as it is. PK, SBombing and SK will exists, and devs effort should be used somewhere else.
Or "unguard" all cities. Allow players to create bases near every city (or even in). Let players guard cities (with militia, mercs). This really works in Requiem. Problem is, that only few cities, maybe one is guarded by players.
Or make from some locations (cities) something between guarded city and players driven city (GM driven city). NPC guards could shoot everyone who is not member, and rest will be one players. Once you get membership, you can kill, robb, thief, SB, SK whatever every other player citizen with consequences made by players. If you troll or PK, they can (leader, GM) order you punishment or even exclude you. Of course that unknown players will not have guaranteed acces, they will have to deserve it (GM could give them special quest, bring something, go somewhere, post your screen from Glow, ...). This could work.