When I launched a browser (Firefox, Safari) I was immediately presented with the YubiKey authentication panel. I wanted instead to manually launch the login and authentication process.

To prevent that automatic login (i.e. to manually choose to login to lastpass and then provide authentication):
LastPass_Icon/Preferences/Advanced/Open Login Dialog When Start Browser = unchecked
LastPass_Icon/Preferences/Account Settings/YubiKeys/Permit Offline Access = disallow

The default setting for the first is checked and for the second is allow. The first item launches the login dialog, the second allows lastpass to grab the master password from the local cache, and opens the Yubikey dialog. My first attempt was to set the first item to unchecked and leave the second at allow, but that had no apparent effect. Took a couple of emails with their tech support to figure it out.

UPDATE: When I restart the computer and browser launches, I still get the Yubikey authentication box. Firefox and Safari are obviously getting my lastpass master password from somewhere, but the remember password settings in the browsers are disabled and I can't find anything in the OSX keychain, so I'm at a loss.