Latin uses several words in the sense of "surrender". Since you specify fighting, I've restricted this to "arms/weapons". Various options are available:

ARMA NOLI UMQUAM PONERE (or in a more English but less mellifluent word order: NOLI PONERE ARMA UMQUAM), which means, "Never lay down your weapons."

Use NOLITE (plural) instead of NOLI (singular) if the command is directed at more than one person.

NE PONAS ARMA UMQUAM or even NE POSUERIS ARMA UMQUAM is a touch more stylish, using a negative subjunctive verb.

Again, use PONATE or POSUERITIS, respectively, if the sentence is spoken to multiple persons rather than to one individual.