Hello Konstantinos,
based on your Captive Portal design you may either use access time value / timeout
![20b5498c676e4ec390bf1faf0c435b71_RackMultipart20170215-43416-olf4wl-Captive1_inline.png 20b5498c676e4ec390bf1faf0c435b71_RackMultipart20170215-43416-olf4wl-Captive1_inline.png](/t5/image/serverpage/image-id/5385i9020E47D3223DAE4/image-size/large?v=v2&px=999)
Or if it is driven by RADIUS (internal) you may change RADIUS group attributes
![20b5498c676e4ec390bf1faf0c435b71_RackMultipart20170215-8708-v0ar6l-Captive2_inline.png 20b5498c676e4ec390bf1faf0c435b71_RackMultipart20170215-8708-v0ar6l-Captive2_inline.png](/t5/image/serverpage/image-id/1058i89C11EE860AD970E/image-size/large?v=v2&px=999)
If you want to de-authenticated Captive Portal users you may do it through Statistics - Captive Portal.
Please find Captive Portal How to Guide under
this link for more details.
Regards,
Ondrej