I don't think you need a spiked collar, as long as your tree has a single trunk and no close-by tree for a cat to jump from. Just wrap a collar of CLEAR polythene about three feet deep round the trunk and the cat simply skids back down when it tries to climb. It worked a treat below our nest box last year. But the polythene needs to let light through or it won't do the bark any good.
If a cat can't dig its claws into something like bark, it can't climb.