How can I disable and re-enable machines in a BIG-IP pool via ssh?

http://askf5.com/ is a good source for all kinds of documentation. You're probably going to need the Bigpipe command line reference sections for "pool" and "node". The document for TMOS 10.1 can be found here:

http://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/bigip_utility_reference_guide_10_1_0.html?sr=16073894

You may want to disable the node complete if it is going down for maintenance*1, or you might want to disable the pool member if you're updating a specific application*2.

b node <node name> down
b pool <pool name> members <IP:port> session <enable|disable>

Be aware that the commands might vary slightly between TMOS versions. There is also a configuration documentation error relevant in this case: http://support.f5.com/kb/en-us/solutions/public/11000/800/sol11845.html.