I constantly receive strange topics like in the attached picture.
Is there any method to protect against them ?
I know this kind of spams. These are human spammers, real persons, there is no way to stop human spamers. Please read this FAQ topic: https://wpforo.com/community/faq/how-to-stop-spam/#post-39862
There is no bullet proof way to stop human spammers but you can mitigate the problem with plugins like https://wordpress.org/plugins/stop-spammer-registrations-plugin/ which does a pretty good job of stopping them before they even register.