What we first tried was to create a filter that dropped all leads with a name beginning with a number. Problem was the spammer was using hexadecimal, so that only reached some of the bad leads. The available filter tools were a bit limited, so we switched to manually finding and deleting them. We were lucky in that there were two fairly reliable fields we could search for that would return a list of 90%+ spammed addresses, so we searched, then manually deleted all the bad records from the returned list. Took quite a while, but we brought it down significantly. I added some javascript and attached it to the form submit on our end, to prevent the form from being submitted through us with bad data, and that effected a reduction, but not complete elimination. It's not an ultimate solution; I expect it will be just a matter of time before that measure falls as well.
Since then we've been evaluating the idea of presenting and handling the form ourselves, so we can write and enforce our own spam-filter rules, then using an API to insert them into Marketo. No reports available from that effort, yet; we're still evaluating the Marketo APIs for usability and fitness for that purpose.
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