To defeat squirrels, your options are repellents, scare tactics, or traps. To protect your tulip buds, use a taste repellent such as hot pepper wax spray. The wax enables the repellent to linger up to a month, despite rainfall. You can purchase a small, handheld pump sprayer, or buy concentrate to dilute, and apply it through a hose-end sprayer. You may still lose a few tulip buds, because the squirrels don't smell the hot pepper; they have to taste it for it to be effective. Look for squirrel repellents at local garden centers and pet stores or from mail-order companies.
Another option is to hang something like a shiny, light-reflecting wind sock. Of course, it scares squirrels only when the wind is blowing, and they soon become accustomed to the wind sock, so it is a less reliable control. But couple the windsock with the hot pepper spray, you'll have a two-pronged approach to squirrel-proofing your tulips.
In some areas you may be able to livetrap the persistent visitors, and move them to a local park or greenbelt area. Check with your state conservation department about the legality of livetrapping. Your local garden center can help in determining the size trap you'll need.