re:  81 aircooled vanagon ...

Have never needed to try either of these ideas, but it looks
like you could either ...

1.  Find a drill bit essentially the same diameter as the
bolt and just drill into the center of the bolt head and on
into the bolt shaft until the part of the shaft that's in
the suspension arm is just gone.  (Just the bolt shaft on
the end where the nut was, of course.)   Looks a little
tough on the front side where there is a strut bar in the
way of going straight in.  A good technique might be to
start with a smaller bit and then move up to the large one.

2.  If you can get the van on a lift or something to give
you clearance underneath you could possibly use something
like a sawzall to just cut through both the shock sleeve and
the bolt on either side of the shock center.  Carefull not
to cut into the shock body as it may be gas pressurized.
Lots of dirt will fall into your eyes doing this, so ...
... but I think this is the easier of the two possible
solutions if you can get the vertical clearance for the saw.

It's obvious the moral to the story is coat the bolt with
antiseize compound when installing shocks.

Note:  "Sawzall" is a trade name of, I think, Milwaukee
tools, but is the name most people use for what is
generically called a reciprocating saw.  These things can
probably be rented, and you'll need a blade made for metal
of course. A picture is at
http://www.boschtools.com/cd-html/s0300305.htm