A fence or wall is the only thing that will keep casual people actually out. Nothing will keep out a determined trespasser.
Under common law, when they are on your land they are trespassing. You can call the cops on them and get them removed. That might fix one or two of them, but others still might come on to the property.
In a rural area, the rules are a little softer. Yes, technically a kid walking across your field or in your patch of woods is trespassing, but the cops will just tell them to leave, sign or not. Same as adults who are clearly not up to no good. IE, the birdwatcher with binocs or the kid with a BB gun isn't going to get arrested, just told to leave. The guy with the hunting rifle or weed will get arrested for trespassing as well as poaching or whatever else they are doing.
Obviously invitees or licensees (UPS guy, meter reader, friends etc) don't count as trespassers, and they properly ignore the no-trespassing signs.
Thing is, laws don't stop people from actually doing anything. They just punish them later. If only politicians could understand that fact...
Note, this is not given as legal advice, just a general comment on how the law looks at private land and incursions thereto.