I had this problem with my '92 K75s a few weeks ago.
What's going on is that the low voltage coming from the dead battery causes higher amperage to flow through the relay, which means higher temp, which fuses the contact points so that the engine keeps trying to turn over. Once you get the battery disconnected and pull the relay, you can break the contacts apart, but by then the damage is already done. If you take a look at the relay you will see that the contact points have melted a bit. That means that if you keep using it, you don't have a uniform surface for conducting electricity. The relay will be more prone to fusing again if used in this condition, etc.
$70 bucks for a new relay was my solution, and I won't fail to take better care of my battery now.