Amrita,
You have to cancel the material documnet like if you post the excise documnet with MIGO, Then in the same transaction code you need to select the cancel material documnet, If MIRO also done, then you need to cancel the MIRO documnet first then material documnet in MIGO.
System wont allow you to reverse these excise documents through FB08, it will allow you to reverse only FI direct postings.
Regards,
Ravi
