וַיִּֽחַר־אַ֤ף בָּלָק֙ אֶל־בִּלְעָ֔ם וַיִּסְפֹּ֖ק אֶת־כַּפָּ֑יו וַיֹּ֨אמֶר בָּלָ֜ק אֶל־בִּלְעָ֗ם לָקֹ֤ב אֹֽיְבַי֙ קְרָאתִ֔יךָ וְהִנֵּה֙ בֵּרַ֣כְתָּ בָרֵ֔ךְ זֶ֖ה שָׁלֹ֥שׁ פְּעָמִֽים׃
И балак'гнев возгорелся против Валаама, и он стукнул руками; И сказал Валак Валааму:'Я призвал тебя проклясть врагов моих, и вот, ты трижды благословил их.
Sforno
ויספק את כפיו, as a sign that he had abandoned hope of achieving his objective, seeing that Bileam was cursing those whom he wanted to be the cursers.
Or HaChaim
ויתר אף בלק..ויספק את כפיו, "Balak became angry… and clapped his hands, etc." He expressed his anger by clapping his hands together.