How can I prevent players from placing blocks on an unmodded server?
Solution 1:
Put the player in Adventure Mode with the command /gamemode <playername> a
Solution 2:
My answer in the linked question is a viable solution for this as well.
Modify the following variable in the server.properties file:
spawn-protection=999999
Then OP yourself and anyone else you trust to place and destroy blocks. There must be at least one OP for spawn-protection to work.
Solution 3:
The answer is simple: if you do /gamemode adventure @a, than none of the players can place or break blocks.
I hope this helped you