from Ohr Somayach website:
As for not blowing out candles in general, the following reason has been said in the name of Rabbi Yaakov Yisrael Kanievsky (the ‘Steipler’), zatzal:
There’s an angel whose name is the same as the sound produced when you blow out with force. According to Kabbalistic tradition, it’s improper to make use of the names of holy angels. Many people, therefore, extinguish candles by hand in order not to pronounce the name of this angel.
Another explanation: The Torah depicts man’s soul as being a ‘breath’ from Hashem. The soul is also compared to a flame. Using your breath to blow out a candle is an ironic gesture, using one ‘soul’ to extinguish another.
I know a rabbi who puts out candles by saying the word ‘Purim,’ forcefully emphasizing the letter ‘P’.