In dramatic contexts, the sequence ou) mh/ of the headword phrase, followed by a second person subjunctive, expresses strong prohibition (Smyth, 1800c (note)). Here the verb is indeed the aorist subjunctive active, second person singular, of lhre/w (I talk nonsense, am foolish); see LSJ s.v. The entry is generated by an instance of this expression (although some manuscripts transmit the alternative spelling lhrh/seis; Dover, p. 26 and pp. 140-1) at Aristophanes, Clouds 367 (web address 1). See on Bernhardy in omicron 918, and further, next note.
[1] Except for the participle, which is fluarw=n (prattling; see LSJ s.v. fluare/w), the scholion to Aristophanes, Clouds 367 (web address 1) is the same.