I totally don't like any user gymnastics just to allow them to post in a form, and it is quite not needed with integration with Wordpress Hashcash or Akismet, for example. Does your plugin do so?
Can it and would you consider it in the future if not now?
If not that, you could implement a honey pot approach. Create a field that is not visible to a human visitor but is "there" nonetheless and visible to machines and require the field to be blank in order for a successful submission yet trick the machine to think something must be there so the machine fills it in.
You can see more about form honey pots here:
Though I hate user gymnastics, your form software was so good I could not resist. I hope you will consider the honey pot alternative.
Thank you for reading!