From the help page of the plugin:
6.3. Assignment operator and Rnoweb completion of code block
...
To completely disable this feature, put in your vimrc:
let vimrplugin_assign = 0
--
EDIT: As of writing this (2019-10-25), add this to your vimrc instead:
6.4. Assignment operator and Rnoweb completion of code block
...
let R_assign = 0