@generaal I've looked into slide captcha source code, and I didn't find any actual server-side check in there. It seems to me that this is client-side verification only, so bots that don't execute javascript won't be stopped by this at all... which is kind a weird, considering this module should be aimed to exactly these bots.
I've actually tested this on your site - I didn't slide captcha, but I've manually removed 'display:none' and 'disabled' attributes from submit button (these were added by captcha javascript), and submitted form without problems. You can check in your back office if there's contact message from my email address. For bots, it's even easier. No javascript code was triggered, so submit button is very visible to them.