Do you really need a loop at all?
A series of set field steps:
Set field [Yourtable::yourFIeld ; "" ]
Will clear all your fields and you simply do not include a step to clear the comments field.
If you are using a looping script that puts the cursor in each field with go to next field, then
exit Loop If [ get ( activeFieldName ) = "comments"]
should serve to exit the field. (be sure to use your exact name for this field as entered in Manage | database | fields.)
Note also that in some situations, using either global fields or a related record for the fields where this data needs to be cleared could either eliminate the need for such a script or it could possibly be replaced by a single delete record step. Much depends on why you want to enable people to enter data into fields only to delete it instead of retaining it in a record.