You could try inline-blocking to save yourself coding a theme layer. Not saying it will work, but you could try this:

span.poster,span.entry-poster{display:inline-block;}

And add !important before the semi-colon at the end if it doesn't work right off the bat. I'm not at all sure if inline-blocking will do the trick but if does it will basically allow you to style both span classes as separate (but still hierarchically correct) block elements without messing the rest of your nearby CSS up.

Do you want to do this to flip the order of comm/poster? That I could try out for you myself, if you want.

ETA: Luckily your style sheet works on the same layout as this community's so that was easy. Ninety's code above works fine to flip the name order and hides the "posting in" text without any inline-blocking (which I never doubted, I just wasn't sure why you wanted to separate the spans) so if you still want to separate them (as opposed to merely styling them as separate entities) I'm pretty sure that, as said above, you will need a theme layer to do that.