I had the same problem with a home built throttle quadrant I made.
(might also be with my X52, but haven't flow the 757 much recently)

What I THINK might be the problem, is that your lever doesn't send the exact extreme position to FSX, but something right next to it.
Combined with how the spoilers of the CS bird work (all or nothing) this translates in them opening again if not completely closed.
If it is this way (I'm not sure!) you can try adjusting the dead zones of your lever.
Also, I believe you can have the handle act as a position switch using the SST software.
this way, you can say the up position is 'retract spoilers' and the down position is 'extend spoilers'
or something like that.

If all else fails, and you have the full version of FSUIPC, you can make lever settings in there, to work the magic.

I hope this helps in any way...