Certified GKH Gardening Expert
You can try root pruning. How much you can do will depend on how big the tree is. You can safely prune up to 1/3 of the roots away at once but make sure to also prune an equivalent amount from the above ground portion as well. You would then need to wait about 6 months before you could do it again.
Depending on the size of the tree, 1/3 may be enough to down size as much as you want, but you may also need to do it a few times. Here is more on root pruning: https://www.gardeningknowhow.com/houseplants/hpgen/prune-roots.htm