Assign unique permissions without breaking the inheritance in sharepoint 2007

I have a disc开发者_运维问答ussion list and need to provide contribute access only to this list for all users with read access(i.e visitors). Basically, all visitors should be able to submit their comments in discussion list, but at same time no contribute access on any other lists of the child site. How can I do this without breaking the permissions inheritance from top level site??

Any workaround/ideas are appreciated..

"without breaking the permissions inheritance from top level site" <-- not possible!

Sharepoint's permission model is based on inheritance. If you want different permissions for one particular list, different from the rest of your site - you will need to break permission inheritance (hence the name inheritance).

Any reason why you don't want to break inheritance?