Left: That's a "dwarf" lilac blocking that window.
Annual pruning can keep it blooming at a more acceptable size (below).
You can keep a lilac's height under control, but still have lots of flowers. To do it you'll need loppers, pruners and fifteen minutes per shrub per year.
Every year, beginning in that year when the shrub first reaches the size you can allow:
Below, the loppers are nipping off an old cane at ground level.
A thicket of weak shoots is at "a."
At "b" is a cane that's coming up from roots well outside the plant's natural center. Take it out, unless you can allow the shrub to become wider.
At "c" is a sturdy one year old cane that is positioned for enough light and room to grow side branches; it's a keeper that will be of blooming age before it crosses your upper limit.
Use loppers or a small saw to remove the canes that have reached your height limit. Hand pruners are fine to remove excess new canes (left of the pruners) and thin, weak growth.
Most spring-blooming shrubs can be pruned just this way. Go to What's Coming Up #86 for a shrub pruning guideline that explains when and how to prune over 60 kinds of shrubs.