Thanks I've seen that suggestion before, it might help. I agree that some users might abuse that though.
For now, users responsibly flagging spam comments (and other comments against the Terms of Use) is our best defense. A lot of comments that get flagged really aren't against the terms, it's up to the Moderators of a particular stream to take action from there if it's the right thing to do.
In the future, AI may help out by blocking more spam comments.
Before you joined, there was a wave of spambots that would hit the fun page with "check out my smexy pix link here" or "Make $52374 a week by clicking this link". Imgflip won that battle but now the spam epidemic is coming from within...