Unlike C-splines, which are controlled via their data points, the shape of a B-spline can only be controlled by constraining its control points. To parametrically set the shape of a B-spline, you must first display its control polygon, either with the CNTRL POLY command on the screen menu, or by entering SHOW_CPOLY ON and selecting the B-spline. With the control points displayed, use same method as for C-splines to constrain them: place point elements on each control point and then constrain those point elements. At solve time, as the control points move, the shape of the spline will also change.

If you assign constraints only to the data points of a B-spline and it appears that they are causing the spline to change shape, it's likely that splitting was on when you placed the point elements on the data points and you are now manipulating multiple B-splines!