I had this problem in my 85' westy.  the cable bent and I could not shut it
off completely.
Instead of purchasing a new cable.
I took off the lever cover ( pull levers straight off and then gently pry
off coverplate)
straightened the cable, and carefully moved the heater lever all the wat to
the left(closed position)....
Now here is where the Mc Gyver comes in:
take a large gauge paper clip, straigten it perfectly, and cut it to the
length of the amount of cable still extending from the closed lever to the
black cable tube to it's left. (approx 2-inches).
This is the partof the cable that is prone to bending
get some strong thread and tie the paper clip to the weakend rod to act as a
splint
continue to wrap and tie the thread around both the cable and paper clip
until the whole length is covered with thread
now take some crazy glue and cover the thread so that it has a nice coating
to support the splint
let dry 30 minutes and try it in the before putting the cover back on

works great!

Malibu tony 85' westy  1.9L 5 degrees BTDC