On a road-flaming ruins of a crashed combine helicopter
Anywhere sandy-ocean/river
Urban-big chunks that look like they used to belong to a building
Underground-cave-in
Desert-fence with nothingness behind it.
Post your best excuses to not let the player go beyond map boundaries.
Giant Dev Textured Walls.
Under construction sign?
A Skybox.
Locked doors, walls, any type of crashed vehicle.
Or a big ‘STOP’ sign and a wall of player clip.
Death.
Firing squad.
Some kind of military blockade complete witharmy trucks and police cars.
Be inspired by Left 4 Dead 2 and use crashed vehicles
Or “Twen’y miles a’ parked cars…”
A giant picture of David Bowie with rape in his eyes.
Cliffs. A little more elegant than a wall.
Be imaginative.
If you want to block the end of a tunnel, and your map takes place in the half life 2 area of time, then block the tunnel with something like a crashed fuel tanker
building/wall/fence.
a mine field!!! with barbed wire with signs saying “Mines Ahead!” Than when they go to far they start blowing up and hurting him 
Fire, Or epic amounts of radioative shit
**Leaches, they always get me.
**
Waist high fences.
Mine fields.
a combine wall will a dispacment.