How can I automatically protect blocks placed by a player in Minecraft?

One particular mod that I use on my server is Grief Prevention. It establishes a protection region for a player the moment they place their first chest, and then after that the region which is protected can be easily modified using the 'claimtool' (configurable, defaults to golden shovel). This also also can prevent players stepping into a protected claim (not by default).

This mod does require craftbukkit or spigot server, and using Cauldron is stable with forge mods.

To use this mod a player places a chest, the first of which will establish a 9 by 9 area that is protected to them alone. To create or modify a claimed region you use a tool designated as the 'claimtool' in the configs. The mod provides useful directions from there. Any area inside a claim is protected from other players breaking or placing blocks. Can't sleep, can't access chests etc. If a player wishes it is possible to 'flag' a claim making it so not only can other players not modify the claim but also can not enter the claim. Literally, it bounces the players out and also prevents teleporting in.

The plugin also supports a number of options for sharing a protected area. For example, you can mark players as builders in you claim thus allowing them to break or place blocks, but not access inventories still.

I can't tell you how to administer a server, that's up to you, but this plugin is very useful and full of commands and options for that. It also supports multiple worlds.

A full list of commands can be found here

A tutorial

https://www.spigotmc.org/resources/griefprevention.1884/

http://dev.bukkit.org/bukkit-plugins/grief-prevention/

If you would like I can either find or make a video demonstrating this. 2 other servers which I play on also use it. It's second only to WorldGuard, which from your question wouldn't be as good as GriefPrevention.