There are two ways to explicitly exclude a frequency from the frequency set and as a candidate frequency in future intermodulation calculations. One way is to select the frequency by left-clicking in the Results window. The dialog box that appears has an option to 'Exclude this frequency from IMD analyses' -- select that option and press 'OK'.
A second method is to select Intermodulation / Exclude Frequencies from the main menu.
The dialog box that appears displays the frequencies that were excluded using the method above. In addition, you may use this dialog box to add (additional) frequencies that will be excluded from future intermodulation calculations.
Note that a frequency that has been reserved but then subsequently excluded will be excluded from future intermodulation calculations -- that is, "excluded" takes precedence over "reserved".