Mavericks Bluetooth : Connections from undesirable (unpaired) devices... Where is the Advanced Bluetooth option?
When you say that you see devices connected to your Mac, are you in the Bluetooth Preference Panel or are they appearing in a different app?

Normally your BT is not in discoverable mode, meaning other devices can't see/pair with it unless it has already been paired.  When you open up the BT Pref Pane, the discoverable mode is automatically switched on.  When this happens, it will list all of the available BT devices it can find (like your FitBit) so that you may pair with it.

If it isn't already on, in the BT Pref Pane, enable the "Show BlueTooth in menu bar" checkbox.  This will add the BT menu in the right side of the menu bar.  Clicking on this will show you the currently paired devices and other basic functions.  However, if you hold down the Option key then click on the menu, you are shown an advanced menu. This will show you if your BT is in discoverable mode or not, which it should only be on if the BT Pref Pane is open.

If there are any devices listed in the menu that you don't want paired, choose that device's submenu and select Remove.