Unlike Minecraft Java Edition, Bedrock servers have no setting in server.properties to disable the Nether. Instead, you have to block access to the dimension in a different way.
Options
Behavior pack
You can create or install a behavior pack that blocks Nether portals from being created or disables their functionality. Upload behavior packs as described in Add Behavior and Resource Packs and enable them for the world.
Command block
With a command block in a loaded chunk you can automatically teleport players back to the overworld whenever they enter the Nether. Example:
/execute as @a[m=!c] in minecraft:nether run tp @s ~ ~ ~ minecraft:overworld
The command block must run in Repeat mode with Always Active enabled so the command is executed continuously.