I never seem to have enough units left to destroy bridges, but it is possible with ground units, it uses up their APs in doing so, you can also bombard by air. There is a symbol in the bar on the bottom of the screen with a destroyed bridge, so I presume you press that as the action.
As for disbanding units, I dont think you can actually disband, but what you can do is feed the component parts of that unit back to HQ.
The Soviet player wants to build a new tank corp, i.e. to get combined tank and motorized infantry types in one unit.
The Soviet player creates the new unit and places it on a hex. When it is created it will have no component parts. He then sets the reinforcement level to priority.
To get some tanks into the unit he selects one or more of his tank brigade units. He then sets the reinforcement level to disband on the tank brigade.
What will then happen is that the tanks from the 'disbanded' tank brigade will feed back to Stavka at the rate of 20 (IIRC) per turn. Starting from the following turn these tanks will then feed out to his other units. In this case, providing no other units are on priority reinforcement, they should feed back to the new tank corp at 20 tanks per turn (plus any other reinforcements available).
Eventually the tank brigade will end up with a strength of zero, but will still be on the map as a shell. I'm not sure if a shell of a unit will effect command, I dont think so, but if it does it might be worth putting under control of a front command or Stavka.
So the unit doesnt actually disappear (although it will be destroyed if attacked) but you can move its component parts to other units via Stavka.
Many thanks for the info, BTW how do you set the level of replacement for units?