arghhhhhhhhhhhhh I fight bermuda constantly....The best thing is to just be diligent with your weeding...Get as much of the stuff out as you can and the minute you see a sprig, pull it.

You might cover the bed right now with clear plastic sheeting and secure it in place (creating a mini-greenhouse). This will warm the bed and hopefully the grass will begin to sprout and grow now. As it sprouts you can remove it rhizome and all this time--and weed the bed. Perennial grasses are persistent so you must, as you know, get ALL of the root. Allowing the weed (grass) to sprout and then removing it in advance of planting is called "stale bed" weeding. Just keep allowing the weed to resprout and as soon it does pluck it out--it is a time consuming method but works. And you can continue the hand weeding into the season, again be persistent.

Here is a link that might be useful: HarvestToTable.com

You can do just what Stephen_Albert said to do, only leave it on for a couple months and it will kill it all without any pulling/weeding. It's called solarizing the soil and if you Google that phrase, you'll find exact directions. Follow them completely (especially tucking in the edges of the plastic) and it'll do the trick on the rhizomes and will kill most of the bermuda seeds and other weed seeds, too.

Just last Saturday, I saw a tip about using two layers of plastic with something between the two to hold them apart, creating an insulating air space. Supposedly, this extra air space will help the soil underneath it all heat up so quickly and severely that it'll kill everything down there in a day. I'll bet bermuda will take a few days, not just one, but I plan to try that method this year. The one-layer method's worked well for me in the past, so I hope the two-layer method will be even better.