There are a few things you can do to deal with this.
Firstly you can stop submissions being made from the free email addresses, so no Gmail, yahoo, Hotmail etc meaning they can only use valid domain names usually associated with legitimate business emails. Or use a data validation service to check the email/domain exists before allowing submission.
Secondly, make sure there is telephone number validation added to your form, the basic validation is native to unbounce but again you can extend this yourself or with a telephone number validation service.
If you’re after a ReCaptcha then I would recommend the Google ReCaptcha API, I’ve not actually used it on Unbounce but tbh it shouldn’t be that hard to implement, you may just need to jig things around a little bit and include a custom validation to Unbounce standard jQuery.validate rule set. https://developers.google.com/recaptcha/intro
I’d also add a couple fields onto your form to start gathering some data on where these leads came from and a little bit of script so you can see what the referring website/source was. Maybe you can do other things to shut down the source of the spam. For instance, we have just blocked Russian traffic from our servers as we have no clients who do any business with Russia and nor do we at this time, it accounted for 1/3 of our server traffic from Russian bots and email hacking attempts. Our pages now serve faster to legitimate users and our spam attacks have halved. If this is a bit much then fall back on Google analytics data and see if you can find the trends, should be fairly easy to spot.
I hope that helps a little, its just the tip of the iceberg but should be enough to solve your problems. Give me a shout if you need any more help.