Since it's banned, do those who circumvent it by short-URLs deserve a spam-flag?
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No. They aren't spam. Downvote them and/or flag for moderator attention explaining what they are; a mod can delete them and take any further action that may be necessary in case of repeated abuse.
Edit: The above assumes that the link is an answer. If it's a comment, you obviously can't downvote and there are no distinct flag reasons, so just flag it. One of the reasons for flagging a comment is "noise" and lmgtfy comments are definitely noise.