is it possible to block a user's access to every server / IP except one specific server once he is logged in by OpenVPN, and how can I implement that?
The other users should not be affected by this restriction.
If you use x.509 certificates for every different user, you are able to assign a unique ip addresse from the openvpn remote ip pool. See option
After this, you are able to use iptables as usual.
If your openvpn is configured to use routing not bridging you can do this by pushing them only a host route /32 based on the certificate they send. The openvpn sample configuration has an example of how you can set this up.