It'd be nice to have a shorted height. Make the picture somewhat prominent. Keep the message short and powerful and leave a link to the explanation. Don't explain. Taking up too much space for the flag is annoying when you repeatedly flag on the same page.
Never use the term "downvote". Use "Flag" as it has more impact.
Tell You flagged them for spam.
Leave a link for flagging criteria.
Say it's a warning.
Tell them that you are not a bot.
Just a picture and 4 clear short points with links will do.
Don't play it nice.
If possible, have different messages for warning spam, nuking spam, Plagiarism.
Good Luck!
Very good points.. in the next few days I will revise it for the third version of my comment against spammers. Thanks.
Happy to help the good fight :-)