Even though the mercury is allegedly sealed within the sword, it's still possible for the darned thing to break and spill its precious cargo.
Mercury is an incredibly fine liquid, is it not? If there's even a smidgeon of a crack or crease in the construct, that metal is going to find its way out.
A year or so ago this weapon was discussed thoroughly on these boards, and those among us who had some engineering tech geek skills explained how hollowing a sword could be done -- and whether a few ounces of mercury would make much of a difference one way or another in the swinging momentum of a sword. At the very least, the smithing process would (realistically) make the sword more fragile and likely to break than a common sword design.
It's a silly concept anyway.
If the DM really wants to get rid of the thing, I'd suggest secretly planning a list of reasonable events that could be snuck into the adventure without triggering suspicion of malice. Don't push them into the situation obviously; blunt manipulation of characters is bound to backfire. But if you've got a list ready, the chance will present itself.
Someone already mentioned sundering, for example. So make sure they run into a really nasty villain who can put that action to best use. Have him break a few other weapons, too.
Sliding stone trap doors? Put the character(s) in a room or dead-end hallway with a panel slowly moving to close them in. They've got to stay in the room for some reason, but they want the opportunity to leave later (you work out the details). Would the fighter be willing to prop open the door with his special sword? ... Crack! Problem solved.
Robbery in the city? Take a bunch of stuff and fence it immediately. The sword will never turn up again. (Or maybe it will, in the hands of someone else.) Theft during the night at the inn? Same thing.
Toss 'em into a sea passage or over a waterfall -- belongings are forever lost into the murky depths. Even if they go diving later, some other creature has picked up the sword already.
Have a dragon sit on the darned thing. Whatever.
And when the original sword is gone and the guy wants to buy a new one, make it very, very difficult to find the materials, skills and time to forge a replacement. ...
(If you want to be really nasty about it, give the character mercury poisoning from the unnoticed contact with his skin over time.)
