There is an `editConfiguration` property of the sketch editor. On that property you can set the `vertexEditMode` to `AGSSketchVertexEditModeSelectOnly` (`.selectOnly` if using Swift). I think that should do what you want:
/**
Specifies permitted operations on the vertices of the geometry.
@since 100.2
*/
typedef NS_ENUM(NSInteger, AGSSketchVertexEditMode) {
AGSSketchVertexEditModeInteractionEdit, /*!< Users can interactively select, add, remove, and move vertices. */
AGSSketchVertexEditModeSelectOnly /*!< Users can interactively only select vertices. Vertices cannot be added, moved, or removed interactively. */
};