As for whether or not to write out numbers in dialogue, that specific topic is covered in this post.
The general consensus seems to be that simple numbers should be written out, and that longer or awkward numbers (such as phone numbers), or numbers used repeatedly, can remain.
The thinking behind this is that people don't say numbers—although, logically, I'm not sure it's clear that people say letters either. But that's a different debate.
The short answer is that there is no rule, but there do seem to be common and less common styles.
I think that in the case of a screen name that includes text and a number, and which is used repeatedly, there could be little objection to using the number.
As a reference, look at the dialogue in the Star Wars script (which I'm assuming is legitimate):
This R2 unit has a bad motivator.
Based on that, and the other answer, I think it would be fine to simply use the screen name exactly as you have it:
As an interesting side note, I found something else in the Star Wars script. The first time that the robots are introduced in direction, their names are given phonetically:
An explosion rocks the ship as two robots, Artoo-Detoo (R2-D2) and See-Threepio (C-3PO) struggle to make their way through the shaking, bouncing passageway.
I'm not sure if this is common or not. I can't really see it adding value to the director. (Aside, perhaps, from pointing out that it's an oh and not a zero in the latter name.)