So you are telling me that even though you are considered strong by them, they still send explorers into your land? I have never seen that before and I am not buying it.
If there is open land behind you and they have knowledge of the path, they will send settler combo's across regardless, but you can demand they leave. We were not talking about that or being at war with you or someone on the other side of you.
I often see them come right up to my border, but they back off. Of course I will not ask them to leave, I will kill them instead, but that is another story. IOW they will not bother your land, if you are strong, just to explore.