Honestly, ff's rarely occur in 6 or so worlds (regular daily battles occur in colo, houston, idaho, juarez most of time, and kansas) a game wide rule that sets definition of multi digs where peeps get punished or goes against server own agreements in place seems like could cause more issues. In past, mods would look at individual situations. Agree with comment earlier that if there is a rule established by inno it should be officially and clearly communicated to all in server.
I dont like multi digs myself, (in my thoughts 6 hours mainly based on feedback of players) - dont think generally good for worlds and silly small battles and hurt feelings. The reality is .. there isnt alot of ff's going on in most worlds, so not a real issue and seems to take care of itself when does occur. Some worlds they have player base to do multiple battles per day (colo/kansas), most cannot effectively do more than 1 battle per day so regardless of how short of time frames defined as multi players themselves dont show up and there is decline in attendance in long run which changes the behaviour of multi digger usually without inno being involved. We should trust mods to evaluate significant problems of same multi digger on a one by one case - when the intent is to disrupt server, harm server, steal forts. But the players themselves are the ones that "punish" multi digs.
Know in places like colo and briscoe there is few one player towns that dig multi constantly, looks like those leaders/teams deal with it well and no forts usually taken so may be annoying but not game changing. In AZ, there are several alliances and kinda up to them to work it out where everyone has rights (would luv to see battles there again!). In houston, saw group multi dig according to 6 hours between (in place for couple years) BUT would hate for them to get in trouble (we certainly will use info with players to gain more support to defend but would hate to see peeps get in trouble)... thanks to those peeps and few on our side, actually heard that was only time they COULD dig and us digging when we did left them with limited choices ..so ... now we ironed out wrinkles and have been enjoying guaranteed daily battles in houston - an example where multi used against us but made us "hear" and happy end result. (SHAMELESS PLUG FOR HOUSTON .. lol . no we are not dead) In idaho, many of players there to just show up for daily battles and dont involve themselves in growing idaho politics .. i suspect one group saw another group taking to digging weird hours while bragging active server where, again the players deal with it .. lose support .. went from good daily battles to little 18v9 battles, dont know but expect multi digs were to try to address that. Probably digging multi not best solution but it may have been "hey listen" like the opposition did to us in houston, and then both sides interested in that world should find out why multi and try to work it out else they end up with silly small battles when before were adequate. The kansas multi dig wars ..yeah glad dealt with.
I dont like multi digs, and really do try to encourage the idea of working towards one battle per day (exception being colo and kansas who can handle more) but think developing game wide rule for multi digs, not really necessary and each world different, and may interfere with dealing with real issues. Handle real issues of chronic multi diggers case by case basis! Should also say seen whole servers die off because of multis (one makes me very sad), but those are old tendencies no longer applicable.