Saying the name of an angel:
The Arizal was very careful not to mention the name of angels in his speech, such as upon giving a Torah Drasha, and considered it a prohibition. Rather, he would say the initials, such as Mem Tes instead of Matatron, and when he desired to mention Samael, he would say Samech Mem. It is however permitted to mention the name of Michael and Gavriel, and of any other name that is used amongst people for naming their children. [Likewise, it is seemingly permitted to mention the name in the midst of a prayer or supplication.]
The name Samael: One who mentions the name of the angel called Samael, transgresses the prohibition of “Visehim Elokim Acheirim Lo Sazkiru.”
 Custom of Arizal, as brought in Shaar Hamitzvos, Nagid Mitzvah in Seder Hakeria, Shaar Hakavanos Drush Alef Tefilas Arvis; Shelah p. 100; Zechor Leavraham 3 Mareches Habayis; Kaf Hachaim 5:10
 See regarding the statement of Zevadya Yishmireini, said on Rosh Chodesh after Hallel: Siddur Admur; Siddur Rav Shabsi [student of Baal Shem Tov] and Rav Yaakov Kopel [brought in Shaar Hakolel 37:9]; Mishnas Chassidim Rosh Chodesh 2:1 [1700’s]; Sefer Chemdas Yamim [1700’s]; Machberes Hakodesh [Harav Nassan Shapiro-1600’s] Shaar Rosh Chodesh in name of old manuscript; Kaf Hachaim 422:57; It is omitted in Siddur Harizal and in Peri Eitz Chaim [Shaar Hakolel ibid] Zevadya is the name of an angel. Some are accustomed not to recite the names Zevadya due to the prohibition in reciting the name of angels. Hence, they say this word in their mind. Nevertheless, Admur writes “And he should say Zevadya..” in order to emphasize that one is specifically to verbalize it. This allowance is seemingly due to that it is also the name of a person in Tanach. [Shaar Hakolel 37:10; See Nefesh Chaim [Falagi] 32] Alternatively, the prohibition of mentioning the name of an angel only applies if one does so for no reason, while if one does so as part of prayer, it is permitted. [See Kaf Hachaim 5:10 that the Arizal would not mention the angels in middle of a “Drasha”] Some Siddurim however avoid this issue by replacing the name Zevadya, with the name of Hashem.
 Shaar Hamitzvos Parshas Mishpatim; Kaf Hachaim ibid