You didn't mention which Windows version you are working with. So, I am going to take the Win2K/XP route on this, since I have Win2K and XP both.
Right-click the Marketing folder, choose Properties -> Security -> Advanced button.
In the "Access Control Settings For" window, you will see each group/user listed that has some type of rights to this Marketing folder. You will have to set this one at a time, unless you have the Everyone or Domain Users group listed.
Anyway, start at the top user/group listed, and click the View/Edit button.
Each specific right this first user/group has will be checked in the Allow column. Things they can't do will not be checked. If there is a Deny permission enabled (probably not, but you never know),it will be checked.
So, you want your users to be able to "add files/folders, modify exisitng files, but not allowed to delete any folders/files". Ok. DENY them this right.
Click in the Deny column for the entries "Delete Subfolders and Files" and "Delete".
Click OK to close the "Permission Entry For" window.
You are back at "Access Control Settings For" window. Select the next one and repeat steps until all users/groups you want modified are modified.
Then, click OK on all windows to close all of it.
That's it.
Test this out first, just to make sure it is doing what you want it to do.
hope this helps
