I have previously written about how the recaptcha system appropriates the labor of users without their explicit understanding and consent, and without an 'opt out' option. To me, forced or unacknowledged labor is an issue of fairness and a desire for an informed and empowered society. Yes, it may only add up to a few minutes a week, but the principle remains the same.
Thus I am suggesting a course of protest action. When filling out a recaptcha you see two words. In many cases it is easy to deduce which is the word there for the user's benefit (to show they are not a spam bot) and which is there to make the folks at Recaptcha money and kudos (by requiring use to interpret a work from a document they are probably being paid to digitize).
When I feel I know which word is an attempt to appropriate my labor (often a non-word like a syllable or number) I instead type the word "slave" in response. If you feel as I do about this issue you might try doing the same. So long as you correctly fill out the word that is there for visual verification your comment will still go through as normal.