Do the walkers killed with a blow to the head / brain, reanimate again?
Short answer: No. Assuming that you smashed the part you have to smash, and assuming that you smashed it sufficiently, the zombie will stay down.
If a blow to the head makes them permanently dead, and if in Season 3 the main characters stay mostly in one place, shouldn't they have killed most of the walkers around that area so that they stop returning "every half an hour"?
Short answer: No. They will never stop seeing new zombies.
Secondary question in detail:
The prison the survivors use is fictional, but it is called the "West Georgia Correctional Facility", which tells us where it is supposed to be located: western Georgia. Atlanta is in the same area. Downtown Atlanta is home to a middling 450,000 souls, but the metropolitan area has a population of roughly 5,500,000 people.
The first five and a half seasons are set in Georgia. Georgia is bordered by Florida, South Carolina, and Alabama. Population figures are as follows:
South Carolina: 4,800,000
Total for the four-state area: 30,700,000
Broadening the scope a bit, the Eastern United States - that is, the states east of the Mississippi River - has a population of roughly 190,000,000 people. That's nearly 60% of the population of the entire country (~300,000,000), in an area that occupies about 25% of the land.
I won't even get into the 36,000,000 in Canada and 120,000,000 in Mexico that might wander into the US.
States by population
Population density per square mile
Before you say "zombies there aren't an issue here", think back to season two. A helicopter flying overhead combined with an ill-timed gunshot led to this sequence of events:
The Kill Count Prior to Season Five:
There are statistics for zombie and human deaths on the show up to the end of season four. This site lists the kill count in seasons 1 through 4; I don't know if their numbers are exactly right, but assuming that they aren't wildly inaccurate, they are good enough for our purposes. I have taken the liberty of breaking down the data and creating a spreadsheet.
The members of the group who are listed by name in the chart have killed 956 zombies and 39 people. Other human characters have killed 229 zombies and 136 people. Zombies have killed 78 people.
The grand total:
1,185 zombies killed
253 people killed
In the comic books, and to a lesser extent on the show, Rick's group are depicted as being among the best fighters left in the world. They have repeatedly fought off much larger forces of zombies and bad guys alike. In comparison, people who aren't in Rick's group are usually zombie fodder.
The first four seasons of The Walking Dead represent perhaps 2 years of time. In 2 years, the best zombie killers around have only managed to kill 956 zombies. This does not bode well for humanity.
How Many People Versus How Many Zombies?
The blurb for the tenth issue of the comic book says:
After the ordeal Rick has endured last issue, he sets out to find safer shelter. More is learned about the zombies that now out-number us 5000 to 1, but when it comes to some things, it's better not to know. Rick begins to wonder if there is a light at the end of the tunnel his life has become. Even if there is, how can he ever expect to make it there?
In Issue #10, Carl is brought to Herschel's farm for the first time, having just been shot by Otis. This corresponds to the second episode of Season Two of the show. The second episode of Season Two takes place about a week after Rick wakes up in the hospital, and a few weeks since the outbreak began.
This means that within weeks of the beginning of the outbreak, only 0.2% of the population was still alive. What does this mean?
0.2% Humans, 99.8% Zombies:
Worldwide: 1,400,000 people against 6,998,600,000 zombies.
United States: 60,000 people against 299,940,000 zombies