You could use helper fields in the Main DOM or xdExtensions to set
conditional formatting on fields outside of the repeating table of your
multiple select list box, but the quickest and easiest way to set conditional
formatting is through an XPath expression in the Conditional Formatting
dialog box.

Try this:
1. Open the Conditional Format dialog box on the field you would like to
make read-only.
2. Select "Select a field or group…" in the first dropdown list and set it
to the node representing the option in your multiple select list box (in my
case this would be "selected" under the "option" group; see later in step 6)
3. Leave the default setting of the second dropdown unmodified
4. Select "TRUE" in the third dropdown list
5. Select "The expression" in the first dropdown list
6. Modify the XPath expression as follows:

If you want the field to be read-only when the second option in your
multiple select list box is selected, add a "[2]" behind the option group.
For example…

Expression before change:  my:options/my:option/my:selected = string(true())
Expression after change: my:options/my:option[2]/my:selected = string(true())

Note: You can add more conditions if your scenario requires it by clicking
on the [Add >>] button and selecting "and" or "or" from the list.

7. Select "Read-only" under "Then apply this formatting:"
8. Exit out by clicking on [OK] on all dialog boxes.

Hope this helps.

Regards,
S.Y.M. Wong-A-Ton