in my slim application I have several forms, and there is no spam protection for them.
I would like to implement middlewares/honeypot, but I am not sure how to implement it. Does anyone have a tip on how to implement middleware? Or an alternative, if possible for free 
You have multiple options.
The simplest solution would be to add a form input field and hide it with CSS. All bots will fill the field with a value, but no human, because it is not visible for them.
Another solution would be to use an external API like the Akismet Spam protection.