I would wait until the temperatures are going to be in the 70's for several days. Cut them off at ground level or thereabouts. Drill holes in the remaining trunks with the largest bit you have, as far as the bit will allow. Fill the holes with 100% Roundup. This will kill the root system and keep roots that you miss from sprouting later on. Otherwise, you will have crape myrtle sprouts coming up 5 years after you remove the shrubs. After several weeks have gone by, then get down and dirty and get the roots out the best way you can.
If you have room to plant the shrub roses in between the dead stubs, why not just leave them? They will quickly rot because you will have killed them with the Roundup.

Roundup will kill roots?? I didn't know that.
Will it not hurt the Roses I want to plant?
Is there any other way that I could make holes in the trunk of the trees? I've never used a drill.
I realize this sounds a bit hopeless...but our lot is mostly shaded and if the Crepe Myrtles were moved...there would be sun for the flowers.
Just a silly question...what if I cut off all the Myrtle tree limbs...and as much of the trunks as I could...dig the ground beside and through the roots as much as possible...and plant the shrub Roses? near the Crepe Myrtle trunks?...in other words, try to ignore the trees roots...or, would they eventually kill the Roses?
Eldo, thanks...your answers' sounds good.

Don't use Roundup, it's not designed for woody plants. Use something that contains Triclopyr, such as Ortho Poison Ivy and Tough Brush Killer. I drilled holes in the roots next to the stump and poured in the Triclopyr directly from the bottle (don't dilute it) and it worked great. I never had a single sprout come up after I cut down my Crepe Myrtle tree. Be very careful if there are other shrubs around the tree you're trying to kill. If the root killer gets into the soil, it can kill or severely damage the nearby shrubs.

roundup works on everything ...

but all you need is to apply it on the green layer just under the bark ...

and it become inert when in contact with the soil ...

same with the stuff joebob suggests ... the inner layer.. i dont know about soil retention ...

and it is used.. full strength.. not diluted.. or.. more importantly.. WHATEVER THE LABEL SAYS ...

use an applicator such as the one at the link ... for precision application.. that way it does NOT affect nearby plants.. nor does it ever touch the soil ..

now.. joebob.. the post was 6 years old.. lol ... do you think they ever got the plants out.. lol ...

ken

Here is a link that might be useful: brand is unimportant.. its the restricted flow ...