Hello :). I’ve found a bug where its impossible to delete rigid body constraint from a softbody, where it works on rigid bodies. Ive made a blend which proves this, here. I’m using blender 2.69.
Do you have any suggestions on alternative ways of making constraints with soft bodies?
My game have a grab system style half life 2, and I ran into this problem by doing some tests, I did not care much because my game not need soft body.
Why not, make a constraint(without python, use the constraint tab, and use linking collision) to the soft body with a rigid body, and apply the constraint in the game to the rigid body object? It is just an idea.
I wish this could work, however I’m using groups, which unfortunately do not carry over constraints from the interface :(. Thanks for the idea though :D.
bumpy bump 
At the moment I’m just using a secondary object which I constrain to the soft body, then parent the object I really want. That way I can unparent it afterwards so it looks like it disconnects. I wouldn’t mind a better solution though, as the rope still doesn’t swing right afterwards due to the secondary object.
I’ll bump this one last time if anyone has any suggestions. Is this a known bug? After this I’ll just leave it lol, its clear its a difficult question.
Hint: Ensure your helper object has no mass (e.g. sensor object). This way it’s influence should be nullified.