Redactor Single Line Mode
In my tests the paragraphize setting did not seem to fully prevent paragraphs from being generated. Instead { linebreaks: true } seems to reliably convert all would-be paragraphs to &lt;br&gt; tags.

If you're really serious about sticking to single lines then you can go even further and block all forms of line breaks:

{
    deniedTags: ['br'],
    enterKey: false,
    pastePlainText: true,
    linebreaks: true
}


The only flaw with this seems to be that linebreaks still show up in the editor if they're pasted in, but they are stripped when the entry is saved.