Well there is good news, and bad news. The good news is that I did some research for you and tried to do the best I could to find the best information I could, even calling tech support for SMC on thier toll free number though I do not own an SMC product. The bad news is that because of the way that SMC is set up, they do not offer the flexibility to unassign ports as you need to do.
Yes, Cisco equipment does have that capability as do many of the other manufacturers in the industry. SMC,unfortunately did not put those sort of capabilities in the design for the VLAN apparently. All sources of information, writte, techie, forum, or otherwise were not useful in finding a solution. I think the tech support man may have been right. It may not be possible with SMC.
I know that this is not the answer that you are looking for and you probably will reject it jsut because it doesnt help you. I hate to tell you but this is the only answer you are likely to get so either assign thepoints or lose the points and never get any use from them.