Hi
There are two things that affect this (Both Below)
If you look at the Depreciation Books, and open the book in question (The one specified on the asset if you have more than one)
Under the general section there is a field called Default Final Rounding Amount, when the value drops below this amount the system automatically finishes the asset
The next field is the Default ending book value, the system will not depreciate below this value
In some countries you cannot have and asset at 0, the laws require that it always have a value, so this field is set to $1 and the system drops the asset to this value
So i'm guessing this will not be the case in your country though you should talk to your finance people to be sure
Set the first figure to the number that the system must close the asset below, you may want it at $1 (Or higher if you want the system to shut the asset below for example $5)
Next to fix the currently open entries:
Create a FA journal and use the Write-Down type to remove the last bit off the asset