Building A Plugin Into Assets Folder

I have Unity Pro and have been building a plugin.

When I build my plugin dll, it is moved to the Assets folder, where it is needed.

If I try to build again, it says that it cannot access the dll. I assume this is because Unity has opened it for use in Inspector. Once I close Unity, I can rebuild my project.

My question is this. Is there a way to make unity let go of a plugin? I tried deleting it from the Editor, but his does not seem to work.

Any Help would be appreciated.