I went through a long process spanning a year before I sorted this out, with several very informative calls to the engineer at Volspec in the UK (Cheapest Volvo parts I have found). Firstly the key mechanism was suspect. However, if you remove the black plastic cover on the forward side of the engine where you will find the start and stop solenoids (little grey boxes with push to reset buttons sticking out on the port side) and turn the key to off with engine not running....and you get a clicking sound coming from one of the solenoids...it's not the key. Next swap the two solenoids around....one is for start and the other for stop. If when you do this and the engine no longer starts you have found the problem. Next I was told to run a +ve connection from the alternator to the stop solenoid on the HP fuel pump whilst the engine was running and if it didn't stop then the fuel shut off solenoid was at fault.....of course that didn't work so I replaced the fuel shut off solenoid, and guess what problem still not fixed! At this point I gave up. The fresh water pump was leaking at this point which was beyond my confidence to replace due to timing belt replacement at same time so finally the professionals were called.
Mechanic found that the 55 amp grounding fuse had blown, and as I a had replacement in my box all was sorted! Ever wished you'd called in the professionals sooner?! Finally I answer your question... the 55 A fuse will be underneath the black plastic box on the front side of the engine. It doesn't look like a fuse at all. It is a solid bit of white plastic with bolt on terminals at either end. The Volvo book says you should never go to sea without this spare part....which I carefully obeyed.....just didn't know when to change it!!
I suspect your stopping problem may be more related to my recent start problem .. the solenoids in that black plastic box...
Hope this is of assistance
SM 140 Sula