How to turn off TNT explosions & Creepers

Control Explosions using Game Rules

/gamerule tntexplodes <true|false>    
This command will only work in the Bedrock Edition of Minecraft.
For Java Edition, refer to the "Control Explosions Using Plugin" section below.

If set to true, TNT explosions are enabled in your Minecraft worlds. This is the default value.

If set to false, TNT explosions are disabled in your Minecraft worlds.  

/gamerule mobGriefing <true|false>    
If set to true, explosions caused by mobs will be enabled. This is the default value.

If set to false, explosions caused by mobs will be disabled. 

This game rule will also affect the ability of mobs to make changes to the world or pick items up e.g. endermen

Control Explosions with server.properties

Edit the server.properties file: Open the server.properties file using the WebFTP editor and look or add the the following settings:

"tnt-explosions" - this controls whether TNT explosions are allowed on the server.
"mob-griefing" - this controls whether mobs like Creepers are allowed to modify the environment around them.

To disable TNT explosions and Creeper damage, change these settings to "false".

For example:
tnt-explosions=false
mob-griefing=false

Control Explosions using Plugin

Unlike the mob griefing, you can't turn off the TNT with the classic gamerule command, you will need to install the plugins named WorldGuard and WorldEdit.

First, you must be sure that you are running spigot, papermc, or bukkit on your server to install plugins.
You can change your game version on the first panel page, this won't affect your current world, so you will keep your data.

You can downoad them on  https://dev.bukkit.org/projects/worldguard and https://dev.bukkit.org/projects/worldedit

Once you downloaded the .jar files for each plugin, open the webftp, and drag and drop these 2 files into the plugins directory.

Then, start your server, and you will now be able to turn off the TNT explosions on your server with the the command :
/region flag __global__ tnt deny

if you want to turn them back, use
 /region flag __global__ tnt allow

Same if you want to turn off Creeper-explosion :
/region flag __global__ creeper-explosion deny

and for all other :
/region flag __global__ other-explosion deny

/!\ Note that these commands only affect your current world, if you have multiple worlds, you need to teleport and run it on each of them.