Lilacs (especially) and other spring-blooming shrubs such as rhododendron need to have their spent flowers removed so the plant doesn’t put its energy into producing useless seeds. Use bypass pruners to snip off the flowers while taking off as few leaves as can be managed. If your rhodies are getting too big, carefully snap off the new fuzzy leaves that are springing up from the old flower sites. This removes new growth, will not hurt the plant, and helps to keep your shrub at a size that fits its site, rather than covering the windows and threatening the eaves.
Removing spent flowers from lilacs and rhododendron lets the shrub put strength back into its leaves and roots rather than unwanted seeds.