This script comes in 6 macros - rolled into 2 script files. The script flattens splines to an X - Y or Z snap point. You select the spline - select the verts you want affected and then use the command. The script creates 6 commands - Flatten_Spline_X - Flatten_Spline_Y... Fatten_B_Spline_X and so on. The non B version moves the verts to the proper location but will not adjust Bezier Handles. The B version will also flatten the Bezier Handles so that the spline is truly flattened. You can use ESC key of Right Click to cancel. The command is only visible when a spline object is selected. If you select the command while not in vertex sub-object level - nothing happens. If you select the command with no verts selected - nothing happens - but you are in pick destination position mode so you must ESC or Right Click to cancel.
Download and place the scripts in the Scripts Folder. Go to Maxscript>Run Script and run the 2 scripts. Go to Customize UI - go to Quads Menu - highlight the lower left Quad on the main menu - then expand the Context Spline Tools Item. The Flatten Spline scripts have a filter flag so they are only visible when Splines are selected - and this location makes them visible with other spline tools. You can look for the script in the MC_Tools Category.
Hope that makes sense. If not - go to the Discreet / AME 3dsmax forums and post a topic. I'm an FA there so I'll respond once I see it.