Sure, we've had some issues like that. Tactical placement of bicycles goes a long way. Also, park the truck to "block" one side of the campsite.
I agree completely with the posts about working with other campers. Many times I will have unused space on my campsite, like behind my trailer... if the neighbor needs a little extra space, like for a tow dolly or even their bicycles, I will let them know - but if someone just goes and uses that space without saying anything to me, then I will get annoyed.
When we first pull into a campground I will try to say hello to my neighbors. Especially when I am hooking up, if my neighbor is near I will make a comment about keeping my lines and hoses on my campsite. This does 3 things - Introduces me to the neighbor, shows him I am trying to abide by boundaries, and also reminds him of where I think the boundaries are.
Depending on the situation, I might "play dumb" (which my wife says is very easy for me) and walk over the neighbors asking if they know where the campsite boundaries are, which then does the 3 things as above.
You need to remember that sometimes people legitimately just don't realize that what they are doing is bothering someone else. Especially with new campers, they might think that a campground is one big campsite for everyone to go and do whatever they want. We were sort of like that when we started "campground camping" - it took a few camping trips to understand how to get along with the variety of different people in a campground.
Most times we end up meeting great people and make arrangements so everyone can enjoy the campground. Sometimes I find out we have a grumpy gus next door, which is fine, he wants to be left alone - which is a good thing for us to know.