Some users get on our server with ftp with a specific username (i.e. guest). The users guest has an own group. The user guest is in the path /usr/guests after login.
How can I lock the user guest that he don't get up to the /usr treee. I tried to change the rights for the ..
But it's not anymore possible to log in as guest.

I was not able to fully understand your question or configuration so the reply will be 'fuzzy' based on fuzzy
knowledge.
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There are many way to restrict access to directories and
subdirectories. One is using permissions (as you discuss briefly) another is the chroot command, which will comletely change the root 'of the tree'.
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Normally, when adminstrators want remote users to have on
certain commands and access they change the root directory
of the users and provide them with a subject of the commands they need in a custom bin directory. So,
instead of trying to restrict users to only a certain part
of a large system; they are provided only access to the
commands they need. This is much more secure, safer, and
easier to manage by far.
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Recommendation: Check out the chroot() command and set up an environment for your 'guest users' using this strategy.