In general, i dont know if aproach of regulations concretely, concept of guarded towns, is not more harmfull than it would not exist.
We can compare 2 long time existing severs, TLA and 2238 and their aproach. On TLA (i dont think that TLA is ideal server, in general, i think that there are good thing, ideas, but i like 2238 more), so on TLA, there is freedom not regulated by NPCs in guarded locations, in fact, your freedom is limited by other players, that could kill you, robb you... . It has that effect, that some players unites into militia and they are guardig that location by themselfs.
In 2238, there are guarded locations, but everybody knows that it is only illusion and there are plenty ways how you can lose your possesion or life and you cant often actively guard yourself (or your friend), because of that regulations. The purpose of this is to help newbies and other players to orient, find way, in wasteland so they can settle in this server. Does this work? When they visit first unguarded city to explore world that they think they know (from Fallout 1,2), they are immediately kicked to their... by PKs, robbers..
So i dont think so, that in general, regulations and protection is always helpful, moreover, balancing of this guarding system exhausting developer resources that could be aimed to adding new content... .
Back to topic, what worst thing could happend by planting some sandbags?
1) absolut block of spawn points.. problem, i agree
2) limitations of entrance to any places.? on other way, this is also opportunity to players to break that defense.
Solution?
1) unavaibility to plant on spawn points
2) town preview
3) weight of each sand bag
4) simple click on sand bag barrier would put you sand bag to inventory (like on TLA, so you can "disasemble" that wall)
5) you should be able to destroy it by using science like any other thing (so you can disasemble and destroy that wall)
6) price ... so you could be able to sell it (players would clean it from ground for money)
7) resource needed to craft
Good thing is that this concept already exist and we can use them in its balanced shape. And many others that are on TLA. It is shame that i (Lordus) have to tell you about features that are more than half year old on TLA server.. Dont you think?
But funny thing is that nobody is concerned by fact that supermutants teleporting fatal firepower is destroying the PvP for periods more than one year, but you are concerned of bag of sand...