I don't no how to turn flames off because i joined this match and i suddenly got these annoying flames. These made lag for me, and very irritating.

Please tell me how to turn them off.

Type in /opt particles

thanks, i forgot how to do the command.

I think you need a modifier to the command, it's a toggle.
/opt particles 1 see the flames
/opt particles 0 remove flames

Type the above command in-game.