At each end of the rope, you should tie a stopper knot. This not only prevents the rope from getting untwisted but also helps to make shibari style rope extensions. Knots typically used for this purpose are Overhand knot, Thistle knot, Matthew Walker knot and stacked Wall knots.
An Overhand knot is thicker and easier to open than the others, which also means that it can get undone unintentionally while tying. A good compromise is doing an Overhand knot on untwisted rope. That version is less likely to get undone unintentionally but you can still open it in case you want to repair the rope.
It is also possible to finish the rope with something non-stopping, but then you can't make shibari-style rope extensions with it.