How can I return a 404/50x status code from a Grails Controller?

Setting the response status with its own statement is good enough. It doesn't look too ugly and is pretty straightforward:

response.status = 404;

I've used this successfully myself and have seen others do it this way too. Since it's just a setter, you can also do other stuff after setting the status. Whichever status you set last will be what the HttpServletResponse uses when it actually sends the response back to the client.

I don't know what version this started in, but in Grails 2.2.1 you can do:

render(status: 503, text: 'Failed to do stuff.')

http://grails.org/doc/2.2.1/ref/Controllers/render.html

response.sendError(404) will work with Grails UrlMappings whereas response.status = 404 does not for some reason. This is useful if you want to render a custom 404 error page, as opposed to just sending 404 back to the browser.

response.sendError and response.setStatus are the only two ways I know of. If you static import HttpServletResponse, then it's not that 'un-grails-like'.