I assume you are referring to false pampas grass or ravenna grass, Erianthus ravennae, as real pampas grass, Cortaderia selloana, is not hardy to zone 5. Erianthus is a warm season grass and so is in dormancy now - applying any sort of herbicide at this time of year is pointless. If you don't want to dig it out, wait until spring and the start of new growth and then apply something like Grass-Be-Gone. This will affect only grasses, so no damage to any other nearby plantings. And FWIW, most herbicides, even very broad spectrum ones like RoundUp, breakdown very rapidly once in the ground and and replanting can happen very quickly, usually within a week to 10 days.

It sure wouldn't hurt to start digging it out now. In the spring, the majority of the work would be done and you could treat the stray sprouts that come up.

If it is Cortaderia selloana you wont have to worry about it coming back in spring anywaysÂ

Instead of killing it maybe you could find someone that would want it and be willing to dig it up themselves.
Jim

Since you want to plant something else in that spot, you'll want to dig out the old stems and root ball. Using herbicides in spring just means eventually removing a dead instead of a live plant.

If it isn't too wet or cold to dig, I second the motion to at least start on removing it now.

I have removed true Pampas Grass before. It's work, but not THAT bad. Be sure to wear long sleeves and gloves. Protect as much of your skin as possible because the grass blades are very sharp and will cut you. Digging really is the fastest and most effective way to get rid of it. Sharpen your shovel! Cut around the outer edges of the clump and then use a SHARP shovel (A sharp shooter works best) or axe and start chopping the clump into sections. The roots are extremely tough at the surface, but they are easier to cut through as you get deeper into the ground. You can also try using a pruning saw. Just be warned that this is slower and will likely ruin your saw. Once you've cut it into pieces, it won't be so heavy to pull out of the hole.(If you have a helper, you might be able to just lift the whole clump out of the hole, thus saving a step. Depends on how big the clumps are, and how strong your backs are.) The residual bits are pretty simple to dig out, or you can use Round-up.
I don't know what your springs look like, but I am usually so busy with planting that I would never get to a big project like this then. Besides, the weather is much more conducive to vigorous work right now. You'll thank yourself a million times if you do it this winter. Pick a bright sunny day and get to it. Good luck!
All this being said, if you decide that digging it out is simply too much for you, start spraying it just as soon as it begins to put on new growth in the spring. Keep it up until the clump is gone. This may well take the better part of an entire season, especially if the clumps are old.

If newbee5's winter is anything like mine he/she will have to wait untill spring because untill then the soil is frozen into a solid block that would need a pickaxe and chainsaw to make a dent.

Ooops! You're so right, fledgeling. You have no idea how hard it is to keep in mind the vast differences in climate in this country of ours! This is why I always freeze when I go home to Mom and Dad's house in Tennessee, no matter what time of year it seems to be.
Newbee, use the next few months to work that back and those arms at the gym! :)

If you end up waiting until spring to dig it up, you might consider potting clumps of it up into anything from quart to gallon pots and selling them for a reasonable price.

Lots of people consider pampas grass (or ravenna grass if that's what you actually have) to be desirable. I have a large clump of ravennae grass that people constantly admire and ask if I have any for sale during my yearly plant sale and garden tour. Thus far, I haven't been willing to dig any up-yet.

You could use the funds to support your gardening habit. :)

Some very good ideas here. You could also try putting an ad on your local Freecycle or Craigslist in the "Free" section offering to give away your Pampas grass. I got rid of two very large Spiraea and Forsythia shrubs last Spring that way. A nice young man came with truck and shovel and dug them up and took them away by 4 pm the next afternoon!

But it would be too late here in zone 5 in Mass already, the surface of the ground is frozen and we just got 3 inches of snow yesterday. Yuck! My gardening season is over....unless we get some warm weather again. :(