Unless he's wearing Batman underwear and a cape, Mark Harvey — who usually dresses for Orioles games in a team ballcap and jeans — would be difficult to pick out of a crowd.

That's why he attracted little attention Thursday as he walked outside Oriole Park, less than a week after Opening Day, when he ran onto the field as the Caped Crusader and was subsequently banned from the stadium for life.

Harvey, 26, of Severn, who goes to the ballpark a half-dozen times a season, was disheartened by the ban: "I've been going to O's games since I was little. I've been at Opening Day for the past five years."

But laws in Maryland and other states clearly allow such bans, which are commonly used to prevent crime and as punishment in cases of shoplifting, assault and disorderly conduct. This week, for example, a Baltimore County judge barred a woman from Walmart for five years because of her role in a fight at a Lansdowne store.

Trespassing laws and court-ordered bans can be difficult to enforce, because offenders can get lost in a crowd, but store owners and prosecutors say the measures are an important tool in reducing crime.

"In the case of retailers, we have the right to refuse service to anyone. It can be very effective," said Joe LaRocca, the National Retail Federation's senior asset protection adviser.

Retailers are a major proponent of criminal trespass laws and bans that are conditions of probation, LaRocca said. Stores lost more than $35 billion in 2010 because of shoplifting, internal theft and fraud, he said, and bans can limit the number of repeat offenders.

Though his organization does not track losses that were prevented by bans, a simple warning to someone caught stealing — "just telling someone to stay out of the store" — may have a significant impact on limiting losses, LaRocca said.

Maryland's criminal trespass laws offer the Orioles and other businesses the opportunity to keep troublemakers away by uttering three simple words: "Don't come back."

Property owners do not have to provide the stay-away notice themselves, said Christopher L. Peretti, a criminal defense attorney based in Largo. Police often act as agents of the property owners, he said.

Chain stores such as Walmart and Target often hand out slips of paper printed with a ban notice to people who are no longer welcome, he said. Often, the only defense to a trespassing violation is convincing a judge that notice of the ban was never given, Peretti said.

In Maryland, property owners can keep people off their land by posting conspicuous "No Trespassing" signs or by notifying individuals, verbally or in writing, to stay away. It is a misdemeanor to violate a trespassing notice, and offenders face up to a $500 fine, 90 days in jail or both.

Even these minor penalties act as deterrents, shop owners say.

Brian Shupe, co-owner of the music venue the 8x10 in Federal Hill, said that in seven years he's only had to tell one person — someone who licked the face of a staff member — never to come back. He said he would prosecute to the fullest extent of the law if a person he had asked not to return did come back.

"He doesn't come back anymore," Shupe said.

More severe penalties can be incurred if a judge imposes a ban, usually as a condition of probation.

On Wednesday, Theresa M. Jefferson, 33, pleaded guilty to second-degree assault for throwing bleach and Pine-Sol at another woman in a Walmart at Lansdowne Station shopping center. She was given a suspended five-year sentence, probation and community service.

Baltimore County Circuit Judge Robert E. Cahill Jr. also ordered Jefferson to "stay out of Walmart." Her lawyer said he took that to mean all Walmart stores, not just the one where the incident occurred.

If she violates that order, defense attorneys say, her original punishment would likely be increased and she could get jail time.