Radak, citing Targum Yonasan: I merely said [Abarvenel - empty] words. I did not do anything!

Kli Yakar: No action is needed. After this lowlife blasphemed Hashem's legions and Hashem, one needs only Dibur, to promise to fight him, and Hashem will finish the matter!

Chomas Anach: He hinted, I did not do anything now - but I will do later! Galyus did a great (evil) matters, to revile Elokim's legions! He concealed his words, so Eli'av would not return to speak harshly to him. (Also R. Yeshayah says that David intended to qualm Eli'av's anger.)