Apple - Support - Discussions - Help overwriting iCal with Cloud ...I did this with the 10.4 .Mac preferences (after update, the name doesn't change) and it worked there.
1) Open MobileMe Preferences, and click on the 'Sync' tab.
2) Ensure that 'Synchronise with MobileMe' to 'Manual'
3) Click on 'Advanced...'
4) Click on 'Reset Sync Data...'
5) A window will appear, saying 'Replace: {All Sync Info}'. Use the list to change this to:
Replace: {Calendars}
6) Make sure that the animated arrow is flowing from the cloud to the mac. You can change its direction by clicking on the arrows underneath the image.
7) Click on replace.
It's not obvious how to do this for only one of the many sync options you have. I think if you switch to manual sync, then uncheck all but the item you want to sync uni-directionally, you can leave the other sync items untouched.