Take the battery to mobile phone shop, they may be able to accept it for recycling/disposal.
Or, yes, discharge it, salt-water bath will do to get it down to zero eventually if you care that much. Then bin it.
Or take it to the beach with a pole that has a nail hammered through it for some laughs, you know what to do.
Or check with your council if they can accept for recycling, but most won't and just say to red-bin them (the ones that do accept them at recycling will probably just send them to landfill anyway).