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Marketo's native forms actually do a really good job of filtering out junk. That's one of the benefits nobody really sees, but it's in there (and you can see an enormous benefit when you compare an actual native Marketo form versus hand-coded forms that also submit to Marketo).
BUT in the case that you are somehow getting robot spam, such as if you're doing hand-coded forms that also submit to Marketo, you can set up a honeypot field in Marketo!
What is a honeypot field? It's a hidden field that robots fill out and a human never would. Robot spam mostly scries your HTML and when there's an empty input, blasts spam into the field and submits. A human can't see the field, and therefore whenever a form is submitted, that field would be blank if a human submitted it.
You can create a custom field in Marketo (call it something like "Z - Comments" or something that isn't obviously a honeypot field if a programmer gets smart), and then create a smart campaign that runs daily or nightly watching for that field to be filled. If a lead ever fills out a form and that value changes from "empty" to "not empty," then you know the prospect is a robot, and you can run flow steps to solve the problem. Add them to a static list, send an alert to the admin, and schedule for a weekly delete of the spam prospects.