If the JVM process is gone, what's using your memory?
The memory still used is most likely to be the filesystem cache. Linux will load all the files that JBoss needs into memory and cache them (like jars etc).
Read the first part of this article. Although it's written with db2 in mind, it explains the effect you are seeing.
Basically, there is nothing to worry about.
I would worry about paging to swap space if it happens, which will show up as si/so columns in vmstat or high disk utilisation, viewed with 'iostat' command.
This arcticle shows how to interpret vmstat, which is also important: http://www.linuxjournal.com/article/8178