We have implemented a form on our website using the suggested way as mentioned here: Forms 2.0 - Raw Form Callback However as this doesn't include any of the maketo2.0 js how can we go about adding a captcha or spam filter?
Any help is welcome,
Well, that's part of making the choice to use a standard HTML form instead of using Marketo's form library with its built-in validation and dependencies. If you use an outside form, you have to use an outside validation library. There are literally too many good ones to list here (and way more bad ones).
If all you want is a ReCAPTCHA, you don't need a separate library, though. You just need to have your developer use the straightforward Google ReCAPTCHA docs (and also set up the webhook on the Marketo server -- without this ReCAPTCHA is useless).
Any search result for "implementing ReCAPTCHA" that doesn't pertain to a specific forms library like Marketo is applicable. The examples on Google's own site are all you should need.