Here are the descriptions of the possible node states from the GUI:
Enabled (All traffic allowed)
Disabled (Only persistent or active connections allowed)
Forced Offline (Only active connections allowed)
In this scenario, persistent means clients with a persistence record pointing to the node address for any pool. In other words, if the node address is disabled, and a client makes a new connection to a virtual server with a persistence cookie or source address persistence record pointing to a pool member on the node address the client's connection will allowed to the pool member with a disabled node address.
If you want to remove current connections you'd need to delete the connection table entries. See 'b conn help' or 'tmsh help sys conn' for command details.
On a related note if you set the pool property 'action on service down' to reject, TMM will reject existing clientside connections when a monitor marks a pool member down.
Aaron