It doesn't sound like a JSFML specific problem to me really.
JSFML doesn't keep any object references internally as far as I know, so when you release the references, anything should work as usual. If by "forcing" you mean "System.gc()", and it works, that proves my point, because still nothing would happen if there was something wrong.
You might want to tweak the gc settings a bit, but that's out of my scope - try the JVM docs.