Hi Koolybop, I believe this is a bug related to special characters within the static map names - I have the same problem with the ones with apostrophe's in their names.
A solution: from Server Admin, export DHCP settings (Server -> Export -> Service Settings... tick only DHCP)
View the generated .plist file in your favourite plist editor (or text editor after converting to text via this command:
plutil -convert xml1 some_file.plist - convert back to binary afterwards)
Remove the unwanted static map entries in your .plist or text editor.
Re-import modified dhcp.plist file.
Thanks for your suggestion, dmare, but I found the answer on the internet. the problem was that one computer had two different entries in WGM. I deleted the computer from WGM and was able to delete the static mapping.