Try holding the bottom of the tsuka with your left hand and striking between your thumb and wrist with your right hand...if the tsuka is not absolutely jammed, this should jar the nakago loose. Remember to hold the sword perfectly upright when attempting this so the blade does not jump out on you. If this does not work you may need a special tool called a nakago-nuki. DO NOT bang on the tsuba with a hammer.

The Tools for Tight Handle
1,500.yen + Postage
The hammer is 21.5cm long.
To stick out the tang from the tight handle.
The hammer is made of white oak. The tool for shirasaya is a white wood, but I don't know the tree. The tool for koshirae is lauan, a kind of tree in the South-East Asia.
 

For shirasaya.

 
For koshirae.

 
The postage by regular airmail;
600.yen for Asia.
750.yen for Oceania, Europe, North America.
1,000.yen for Africa, South America.