I use vim (Actually gvim on windows) as my main text editor. In my work flow I have to copy sentences to/from various external sources, therefore I use clipboard=unnamed to
Try this:
:vmap p "_xP
vmap means to make a mapping that only applies in visual mode.p is the key to create the mapping for."_ is the black hole register. This is used in any situation where you want to delete text without affecting any registers.xP means delete the selected text, then paste before the resulting cursor position.