Clear session on FortiGate

1) Clear filter:
diagnose sys session filter clear

2) Add new filter:
diagnose sys session filter src 192.168.1.110
diagnose sys session filter dport 80
diagnose sys session filter dst 192.168.0.1

3) Check applied filter:
diagnose sys session filter

4) Remove filtered session:
diagnose sys session clear

Source: forum.fortinet.com