Use Release to destroy the form and free its associated memory. 

Release does not destroy the form until all event handlers of the form and event handlers of components on the form have finished executing. Release also guarantees that all messages in the form's event queue are processed before the form is released. Any event handlers for the form or its children should use Release instead of Free (Delphi) or delete (C++). Failing to do so can cause a memory access error.

System::TObject::Free 

Destroy