Instead of trimming, have you tried using the split command?
1) Create a construction line that intersects the spline where you want to trim
2) Use the split command to split the spline (notice it doesn't move, rather it creates a spline point)
3) Now that the spline is split, delete the portion of the spline that is no longer of interest
4) Notice the remaining spline doesn't move or change shape
I know we have the sketch split command in R2009, I'm not sure if is available in earlier versions. I hope this helps.
Hugh Henderson (Autodesk)
Inventor Simulation SQA Engineer