Any peer dependency of a package cannot be excluded when requiring that package.
e.g. Using package react-select, which has a peer dependency on react, the following command will always include react in the bundle, despite the fact that it has been excluded with -u. -i and -x behave the same.
browserify -r react-select -u react -o bundle.js
This was tested with version 13.1.1 of Browserify. You can find an example using Gulp with the Browserify API in browserify-exclude-test.