You can do this if you use a plugin manager like Vundle or Pathogen, which will keep the plugin in its own directory underneath the ~/.vim/bundle/ directory.
In that case, just find out the runtimepath of the vimacs plugin with the following command:
set runtimepath?
Let’s say it’s ~/.vim/bundle/vimacs.
Then, put this command in your .vimrc:
set runtimepath-=~/.vim/bundle/vimacs
To load vimacs, just comment that line out and relaunch Vim (or source your .vimrc).