I have heard someone asking if he can mention the name of HKBH and then he said why is there such a rule that he can't... I never heard such a thing, here is his question: "Should we not say the name at all, not even in prayer? Do we say Hashem in reference to Him because it's more respectful, and then say Abba, G-d, El-him, etc in prayer?"
I found this site which deals with this.
The gemara (Megilla 15a) presents the following points:
Aronah blessed David - "Aravna said to the king (you should find favor with Hash-m)";
Daryavesh blessed Daniel - "Elokecha... will save you (from the lions)"
The pesukim (in Shmuel II, 24:23 and Daniel 6:17) indicate that the speakers, a Yevusi and a Mede (I think), used Hashem's name.