I don' think its possible to delete those callbacks (without involving APIs).
We had certain callbacks created in our callbacks queue as part of testing the IVR. Now we don't actually needs to call those numbers but we can't delete them either. So what we did was -- logged in one test agent to that callbacks queue and agent started to receive callbacks. we disposed them off manually by either calling and disconnecting the call before it connects or just clicking 'End Callback' and disposing it off.
This worked for us.
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Cheers,
Sajid Abbas Malek
Al-Futtaim Technologies - Dubai
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