Hello, and thank you for reaching out. Unfortunately, the spline has been like that for as far as I can remember. The only way to hide them from the viewer is through the scene hierarchy, which, in that case, will also hide the assets assigned to it. One thing is for sure, you will not see the spline when the file is exported. If you wish, you submit your suggestions/ideas on the Twinmotion roadmap page. The dev team often look at those 
MC