"Not by us"
अनुवाद:हमारे द्वारा नहीं
You would say "न कि हमारे द्वारा" only when you have something more to the sentence to support it. You would use this to specify that this is not by us, but rather it is by you or somebody else as in the sentence "यह तेरे द्वारा है, न कि हमारे द्वारा". Just saying "न कि हमारे द्वारा" by itself sounds odd.
Actually, this is phrase rather than a full sentence whose whole meaning depends upon the context. Hindi language in several times behave like Japanese or Spanish where people omit subject, verbs or other words during conversation. In written form sentences must have subject and verb but in Conversations there is a sort of flexibility due to previous context. Such phrases can be seen in written dialog also. For example : "तुझमें रब दीखता है" is short form of "मुझे तुझमें रब दीखता है" (I see God in you.). depending upon context "हमारे द्वारा नहीं" can imply "not done by us" or something else. It depend upon context mainly.