Ragdolls should clean up by themselves. Look for console commands starting with nz_, they change stuff in this gamemode. nz_ragdolltime defines how long it takes for ragdolls to disappear. I have gotten multiple reports that they don't disappear, but kept testing it for myself and it always worked.

I'll try this as soon as I can. Thanks for the reply.

So after testing the ragdolls do not disappear for me still. I am on round 15 and the ragdolls from round 1 are still there. Any ideas?

Check other addons. I have no clue why it won't work for you, since you should even be able to read on the code that it should:
https://github.com/Zet0rz/nzombies/blob/master-workshop/gamemodes/nzombies/gamemode/ragdoll/cl_zombierags.lua#L41

Maybe an addon has set Serverside ragdolls instead of clientside?

Whenever I get home I'll try and figure it out. I think I know the addon causing the problem.

Okay, so even with everything disabled but NZombies and the map I was testing (cs_office night version) I still have the problem where corpses will not dissapear. Odd bug, I guess.