Player confidentiality is protected within the tournament site, so I doubt you'll get any info without a subpoena. Best suggestion is to reach out to the moderators by clicking "send feedback" link at the bottom left. Provide your screen name as well as your harrasser's screen name. They will probably want screen shots as proof of harrassment. After a review of the facts, the moderators will advise and take action as appropriate.
If this harrassment is happening outside the tournament (i.e., random opponent), then you should appeal to Zynga: http://newtoy.zendesk.com . Although I suspect they will also be unable to give you personal information.