Hi Greentea,
I understand that you would like to disable some selections from a Multiple-Selection List Box control, and unfortunately I don't think that this possible out of the box. From a blog post by Greg Collins, he was able to achieve hiding check boxes, which is quite similar to disabling controls, by manually hacking/modifying the XSL file when you export your form's source files. Another thing I'm thinking of is copying the repeating data onto a repeating table on your main data source. By doing so, you'll be able to manipulate data better and disable the check box control this way, though this approach will require you to use code. Or you may want to check out qRules if you wish for a codeless way of copying repeating data. Hope this helps.