What's a writer to do?

Today was a bit of a slow day at work (thank goodness!) which allowed my mind to wander often, and I found myself writing bits and pieces of dialogues and descriptions to 4 previous WIPs on a notebook I always carry for such events. The more I wrote, the more ideas popped into my mind. And sometimes dialogues and ideas for two different stories would pop at the same time. So I'd have to try to write one, and hope that while doing that, I'd not forget a thing from the other one. At one point I couldn't seem to write fast enough. Quite scary. *lol* Anyway, tonight I was trying to stick to the Candace Havens’ Fast Draft workshop, but the need to type what I had on the notebook was greater. So after wasting an hour before the screen without having typed a word, I opened the files of those other stories and typed everything I'd handwrittenand, plus added a few new things that popped into my mind. If I put together everything I wrote, I think I did like 5 pages (single spaced). After doing that, and feeling satisfied with the progress of the other stories, I closed those files, re-opened the one I've been working for the Fast Draft workshop and tried to concentrate. However, those characters from my previous WIPs wouldn't let go. They fought for my attention, trying to make me continue writing their story. No matter how hard I tried to block their voices and focus on the new story, their voices became more intense. So now I'm a quadary. Should I ignore them and go on with my new story for the workshop? Or should I put on the backburner the story I'm working on for the workshop? Have you ever been in a situation in which you consciously want to write something, yet characters from other WIPs or from a completely new story keep intruding? How do you choose between them?