Kyohack/B2W2eng — pull request #780 by Elouj (2012-08-06 15:44:53+00:00)
Title: Update 3/0000/0456.txt

Keigo ftl

This sentence wants to say:

I'm sorry, but I don't want (you) to use that tool.

つかう（use) + ない（not） = つかわない
つかわ is a conjugated form, which is used with ない.

Literally, it says "(I) want (you) not to use that tool".

Don't delve into thinking too much about the meaning of ほしい here

～ないでほしい is just a very very formal way of saying ～ないでください. It's Keigo. You can tell the sentence is already very formal from the use of もうしわけない, thus the use of ～ないでほしい is very natural here and keeping the formalities in all parts of the sentence consistent. もうしわけない doesn't go that well with てください

Another alternate and equally formal version of ～ないでほしい is ～ないでもらいたい