I do not use OVPN.
On terminal paste this, using one OVPN real interface name
:put [/interface get <ovpn-test>]
Search on results how the wanted field are called, for example "...;tx-byte=541654;...".
Replace tx-byte on following script with the corect name:
probably working code
/interface ovpn-server remove [find where ((uptime > 00:01:00) and ([/interface get $name tx-byte] = 0))]
If you use this on scripts, DO NOT DECLARE any variable called "name" on the script...
I can not test it, so no warranty that works,
but is nice if you write is work or not for other users.