TMSH Command

Try this:

tmsh modify /ltm pool local_pool members delete { IP:port }

As an alternative to 'delete' I´m using the 'replace-all-with' option:

[root@bigip171:Active:Standalone] config  tmsh list ltm pool test_pool
ltm pool test_pool {
    members {
        10.131.131.203:80 {
            address 10.131.131.203
            session monitor-enabled
            state down
        }
        10.131.131.204:80 {
            address 10.131.131.204
            session monitor-enabled
            state down
        }
    }
    monitor http
}
[root@bigip171:Active:Standalone] config  tmsh modify ltm pool test_pool members replace-all-with { 10.131.131.201:80 10.131.131.202:80 }
[root@bigip171:Active:Standalone] config  tmsh list ltm pool test_pool
ltm pool test_pool {
    members {
        10.131.131.201:80 {
            address 10.131.131.201
            session monitor-enabled
            state checking
        }
        10.131.131.202:80 {
            address 10.131.131.202
            session monitor-enabled
            state checking
        }
    }
    monitor http
}
[root@bigip171:Active:Standalone] config  tmsh save sys config
Saving running configuration...
  /config/bigip.conf
  /config/bigip_base.conf
  /config/bigip_user.conf
[root@bigip171:Active:Standalone] config  tmsh run cm config-sync to-group device-group-failover