While editing spline, I need to make one of the vertexes to be an absolute corner. I.e. bezier control points should be in zero distance from the vertex.
How to do that?
You could try creating an actual vertex of the corner type while you are first creating the spline.
I don't know of a way to change a vertex from bezier to corner after it has been created, however you can use the refine tool to create a new "corner" type vertex right next to it, and delete the old bezier vertex.
Go to vertex mode, select the vertex that needs to be changed then right click on it and select Corner from the menu. To convert it back to bezier then just select bezier again.