To disable/Grey out the "Send To OptOut Members" section while sending an SMS, please follow the below steps:
GoTo Setup>>Manage Users>>Profiles. Here, then select the appropriate profile/s for which you want to disable the "SMS OptOut" option.
Once you select the appropriate profile, scroll down to "Enable Custom Permission" section and click on "edit" (refer below screenshot):
Once you click on "Edit", select the "Allow to Change OptOut" permission from the "Enabled Custom Permissions" section and click on the "Remove" button to move the selected permission under the "Available Custom Permission" section and save it (refer below screenshot for the same)
Once you save it, you can go ahead and try to send an SMS and you will find that the "Send To OptOut Members" section is greyed out (refer below screenshot):