I shared one way to handle curtain tie-backs that are too long awhile ago, see here. I decided I didn’t like that “fix” because I couldn’t see the tassel.
Then I came up with another idea…
Tie a knot!
I tied a knot a few inches from the end of the curtain tie-back. It still was not as snug as I wished. I made it a double knot and liked the length better. I even like the appearance of the tie back with the double knot!