Note: The following method is for a finite area.

1.13+

Command (obviously adjust the coordinates for your needs):

/execute as @a[x=0,y=123,z=0,dx=750,dy=200,dz=750] at @s run tp @s ~ 121 ~

• x=0..y=..z=... - Corner 1
• dx=750..dy=..dz=... - Corner 2
• ~ 121 ~ - Location of player, but at Y 121

1.8 - 1.12

Command (obviously adjust the coordinates for your needs):

/execute @a[x=0,y=123,z=0,dx=750,dy=200,dz=750] tp @s ~ 121 ~

• x=0..y=..z=... - Corner 1
• dx=750..dy=..dz=... - Corner 2
• ~ 121 ~ - Location of player, but at Y 121

1.7-

The selector arguments dx, dy, and dz did not exist pre-1.8.

My method uses scoreboards for detecting position. Since with scoreboards you can execute if a score is in a range.

Note: This will work in a infinite area, and may cause less lag then some other methods

Hovering your mouse over italicized words will give you a short explanation/tooltip

Important: You must be in a Minecraft version of at least 1.13 for this method to work

Preparation

In Minecraft, you need to create a scoreboard objective with a dummy critera. You can do that with the following command.

scoreboard objectives create ypos dummy

After Preparation

Next, you need to have in a Repeating Command Block or a Ticking Function a command that gets the Pos[1] of all players/entities and stores it in our ypos scoreboard. The following command will accomplish that.

execute as @a at @s store result score @s ypos run data get entity @s Pos[1]

If you want it to work for all entities, replace @a with @e. Next, you need a Repeating Command Block or a Ticking Function with a command testing for players that have a ypos score of 122 or more and teleport them to 121 or lower. The following command accomplishes that.

execute as @a[scores={ypos=122..}] at @s run tp ~ 121 ~

Again, if you want this to work with entities, change the @a to @e

Summary

This method stores the ypos of every player/entity in a scoreboard, then checks to see if the ypos value is greater than or equal to 122 and if so, it will teleport them down.