Probably by using a template.  Create your canned response and save as a
template.  When replying, select the template.

Open Outlook and then use its help to search on "template" or
"stationery".  After saving it (use Save As and pick "Outlook Template"
as file type), and when composing a new message, use the Tools ->
Form -> Choose Form and pick the Personal Stationery category.  If you
plan on doing this a lot, you could customize the new-mail window's
toolbars to include the Choose Form button (under the Tools category).

Alternatively, you could define a signature and insert that when
composing a new mail.  You can even specify a template for a signature
to combine them.

move the messages you want to reply to to a new folder.
create a reply template.
create a rule to reply using the template.
run it now on the folder where the messages are.