Successful treatments vary with the kind of shrub. This one can be pruned down low at end of winter and will even flower again the same year on the re-growth.

I have maintained mine at 6-7 feet for more than 40 years by cutting the largest and tallest stems at the base. If it is terribly overgrown you would not want to cut more than 1/4th or 1/3re this year.

I hate them so much I'd cut it to the ground... but thats just me. They throw so many seeds you should have tons of seedlings to start over. I 2nd the advice to take out 1/4th to 1/3rd each yr till you get them where you want them. It took me 3 springs to get 10 ft spindly Lilacs down to 6 ft and full and bushy but it did work this spring finally when I got fullness and flowers.

I cut the thin, branchy growth at the top of stick/stem, back towards the stick. I do this in spring. It makes better twiggy growth with more branching for more flowers. The ROS blooms on new wood each year.

I sometimes let new growth from roots come along to fill out, maybe get a better stick going than the present one, go for more bushy form.

I guess you have to tell us how high your stick/stem is, before it goes out into spindly branches. Is there more than one stem, bushy base instead of tree form?

I find them very adaptable, flower well even when tops are pretty much cut back to the stick/stem each year. I keep mine short, 4ft, so they fit where they are planted. I work the bed around them for annuals each spring, so have not had much seed sprouting to deal with. Any height and they throw too much shade.

If you have more than one, cut one hard on top next spring, see how it does for you. Might send some side growth out, new root growth. You want to trim before leaf growth. Mine don't seem to resent being trimmed hard, just are slow starters to leaf out in spring.