Unfortunately, squatting does occur and often results in very sticky situations for property owners. Navigating such uncomfortable territory is a bit tricky and can be unsafe when not handled appropriately.
In fact, it is surprising illegal to shut off utilities and electricity for squatters. Threats and intimidation are also illegal tactics and breaking the law will only make matters worse for already disgruntled property owners.
If you own a property which is currently vacant, be sure to check on it regularly or hire someone to do this for you if you are unable. Property management companies are more than willing to look out for their owners by performing routine checks on vacant properties.
In the event that squatters are discovered, it is best to notify the police and contact your attorney immediately. You want to stay ahead of things to the extent that is possible and it will be important to proceed legally. In addition, documentation is going to be key. Copies of police reports and documented calls to the authorities as well as police witnesses are also important. Your attorney will be especially helpful in the event that you do not feel the police have handled the matter entirely or appropriately.
Each state has its own laws and protocol for legally dealing with squatters. Paperwork varies from state to state and you might need to include a Notice of Ejectment. Ask your lawyer for more information regarding eviction processes and procedures.
If your situation is deemed a "civil matter" and a judge rules in your favor, the sheriff will likely perform the eviction on your behalf for a fee. Once again, the rules vary from jurisdiction to jurisdiction, so educate yourself on local laws and procedures.
Above all, document as though your life depends on it. Maintain records of all communication with the squatter(s), copies of any forms delivered to them and maintain a professional approach.
In extremely unusual situations, it might even be appropriate to offer the squatters a lease arrangement. In the event that this is doable, you would then have formal, legal rights to evict.
At the end of the day, it is unlikely for owners who have hired an experienced property management company to encounter such matters.