Hello mabr,
I suggest trying to add a bytes value. inactive 60 1024 for example. Meaning that if within 60 seconds less than 1024 bytes are sent, the connection is terminated.
See also --inactive on this page:
https://openvpn.net/community-resources/reference-manual-for-openvpn-2-6/
Note that clients may choose to autoreconnect.
Kind regards,
Johan