We've also been seeing this problem recently. Everything was fine for so long, and suddenly over the past month an incredible amount of spam has been coming through. We submitted spam logs to the plugin author and after 1 week of not hearing anything I gave them a call.
They said they're aware of this problem and are working internally and with WordPress to try to get it fixed. Unfortunately, they didn't have an estimate on when they think that'll happen, but they said to keep an eye out for a plugin upgrade that hopefully will include a fix.
Meantime, we're going to have to switch plugins until this can be resolved. Disappointing because it's an otherwise great plugin.