delete dll

I make a call from external dll

SetPluginUnload alwaysoff
SetOutPath "$TmpDir"
File "${NSISDIR}\Proj1\MyDll.dll"
System::Call MyDll::MyFunc(t) t(r3).r5'
SetPluginUnload manual
System::Free 0
Delete "$TmpDir\MyDll.dll"

but mydll.dll is not deleted. I try to delete it manually, but it can't be deleted until installation application is not closed.
How can I delete this dll before I close installation application?

don't use SetPluginUnload alwaysoff, or add a call to your plugin that does nothing like the call to system::free 0