MyBB Community Forums

Is there any way I can disable all the forum plugins at once

You can turn off all plugins via ACP > Configuration > General Configuration > Disable All Plugins = Yes

Notice this does not truly disable the plugins, just stops any plugins from being ran.

If it is because you are unable to access the Admin CP to perform this, open ./inc/init.php and find:

define("TIME_NOW", time());

and replace with:

define("TIME_NOW", time());
define('NO_PLUGINS', 1); 

Note this is temporary, it stops the plugins from being ran just link the option above.
Note if you do have access to the Admin CP please use Euan T.'s solution instead.