There is a recent thread requesting old cylinders
(Sales would normally require the cylinder to be in date but this is a bit of a special case and they are not intended for reuse)
http://p8ntballer-forums.com/threads/old-air-or-co2-tanks.157336/#post-1428929
For disposal purposes there can sometimes be people who want a out of date cylinder, eg I have seen student paintball clubs using old cylinders that have been drilled out so that they can use them as dummies for publicity and comply with rules that do not permit compressed gases on university grounds
Other people may like them as paperweights, or a training / instruction prop
E.g. A cutaway demonstration cylinder
The main priorities with any disposal is the safety factor of any remaining pressure, and the responsibility of ensuring it won't be erroneously reused
If it is to be drilled or cut then ensure it is empty and the regulator removed - with care as unless it leaks there will be some residual pressure
I would not be so keen on putting it in the bin without removing the regulator, but council dumps (aka recycling centres) can be used, but go down and ask the staff.
I have seen many dive tanks dumped at the local tip and they are put aside
If it goes in standard rubbish and landfill then it is a risk, if it went into a furnace there is the risk of explosion. If it went into a special 'green' furnace I visited once then there will be an explosion, but safely and controlled by the computers. When I was there I watched 'hazardous waste' from the local hospital go through - included oxygen and various gas cylinders - there were a few noises, flashes etc and the dials flapped up and down.
