The usually recommended method to disable is to add this line to your ~/.vimrc file:
let vimrplugin_assign = 0
But unfortunately it doesn't work (at least for me).
When you go through the .vim script files installed by the plugin, the answer lies in lines 2844-2847 of common_global.vim file in the source code:
" Replace 'underline' with '<-'
if g:R_assign == 1 || g:R_assign == 2
silent exe 'inoremap <buffer><silent> ' . g:R_assign_map . ' <Esc>:call ReplaceUnderS()<CR>a'
endif
So the real solution is to add this line to your ~/.vimrc file:
let R_assign = 0
An easy one-liner to do the job for you (if you use a Unix-like system):
echo "let R_assign = 0" >> ~/.vimrc
Do not forget to put two directions as ">>" in order to append and not replace.