Legal Question in Real Estate Law in Illinois

As long as it is on your property and as long as it does not get in the way of your neighbor, visually, or physically you can.

If they get into an accident because the boulder is blocking their view of traffic, their insurance company will have stern words with you. If you put the boulder so close to the driveway that a reasonable person (a judge or jury) is not able to use the driveway, you will have problems.

As your property is next to your neighbor's, theirs is next to yours. Be careful about starting a spite war. Two can 'tango'. I understand the temptation but you must be strong.

If it is on your property and does not obstruct his vision or the use of the driveway you should be fine.