Planarize spline and line and then check for intersection using intersectwith method... If it returns point coordinates, it means there is intersection... Then, I suppose trimming will be possible - not tested though...
Intersectwith method :
(vl-load-com)
(vlax-invoke (vlax-ename->vla-object (car (entsel "\nPick LINE..."))) 'intersectwith (vlax-ename->vla-object (car (entsel "\nPick SPLINE..."))) acextendnone)
The main problem here is how to planarize 3D entity - SPLINE with 1D entity - LINE... Well You could try using OSNAPS : command MOVE, pick point on SPLINE with "_nea" OSNAP and as destination point pick point on LINE with "_nea" OSNAP... If SPLINE is 2D they will be planarized, but if it's 3D, I suppose you could be able to trim one side from intersection point and the other side should remain IMHO...
HTH., M.R.