I think he is asking if he can set an account/certificate expiration similar to how Linux has "usermod --expiredate" or how Active Directory has "Account Expires."
The answer is this is not provided directly in the web administration portal right now.
If you are using an external authenticaion source such as LDAP and the account has expired on that then the user will no longer be able to login. However, their existing VPN user certificate will still be valid and existing VPN connections will still continue.
OpenVPN AS does have expirations applied to the certificates issued but all of them are set to be valid for 3,650 days with no option for the administration to change the length.
But an expiration can still be applied using the sacli script by establishing a Linux "at" job.
For example, if you need OpenVPN AS user "consultant" expired after 1 week, you can do the following over ssh to the OpenVPN AS server:
at now + 1 week << EOF
SACLI=/usr/local/openvpn_as/scripts/sacli
ID=consultant
$SACLI --user $ID --key prop_deny --value true UserPropPut
$SACLI --user $ID RevokeUser
$SACLI --user $ID DisconnectUser
EOF