Is it possible in Marketing Cloud (or in the email coding) to prevent recipients from forwarding an email? For example, when sending an event invitation cannot be transferred.
No, most of the email senders use SMTP which separates the sender from receiver leaving almost no control with the sender.
The only way you can prevent forwarding by applying a policy on the email or encrypting the email but these are not features of the email but the email clients receiving the email. A simple header is added to the email and the email clients enforce this behaviour.
If the question is about specifically about preventing something like an event invite to be forwarded, you can use a one-time-use link as an RSVP and tell the receiver that not to forward this email as it is unique to him, something similar as Go-to webinar links.