If you only want admin users to be able to send bulk SMS campaigns, you can change the permissions on the bulk SMS button to prevent other users from using it.

1. Go to Setup > Customisation > Modules and Fields

2. Open the module where you want to limit bulk SMS access (e.g. Leads/Contacts/Deals)

3. Go to Links and Buttons

4. Hover over Send Bulk Smooth SMS and click Set Permission

5. Remove any user profiles that shouldn't have access

6. Repeat for other modules