First, I wonder if you have talked to the neighbor to explain why the markers are there and ask him to stop removing them. If you haven't, that may be a better solution than a legal one.
It sounds to me like the neighbor is pulling up the markers because he perceives them to be on his property. I don't think the police or sheriff will get involved in a private property dispute unless there is a court order being violated. Because it doesn't sound like there have been any personal confrontations involved, this probably doesn't rise to the level of harassment for which most counties in WA have made getting an anti-harassment order fairly streamlined.
The only way I can think of to get the neighbor to stop is to file a quiet title action against the neighbor, combined with a request for an injunction - a court order for the neighbor to stop pulling the markers. Whether you want to or should take such a step will depend on a lot of factors, including your desire to spend the money on litigation and whether there are any facts that may indicate your neighbor has a claim for adverse possession as to the area between the fence and the markers.
The foregoing is not intended as legal advice, but solely as general information. No attorney/client relationship exists between the poster and the answering attorney.
Removing survey stakes is a crime per RCW58.04.015. The removal could be reported to the sheriff's office or the city police if the property is in a city. I wouldn't expect the police to put this on the top of their agenda, however.
If talking to the neighbor doesn't work, and a lwasuit is too expensive, maybe reporting the removal as a crime would get you some satisfaction.
There is no attorney client relationship between the persons who asked and answered this question. You should always consult an attorney for specific legal advice.
It is a misdemeanor to remove a survery stake but unless you have photo of them in the act it is hard to prove who actually did it. It is also a very good idea to talk to your neighbor about what the problem is. I suggest that you obtain legal advice regarding your boundary line issues. You will need to clear them up before you want to sell your property. Moreover the statute of limitations for adverse possession is 10 years which may be a pertinent fact.