The virus currently only still exists as samples in two freezers in two labs (known to the scientific community). These days I think that that is overkill even for research purposes for this pathogen, what with the genome sequenced and the ability to synthesize arbitrary sequences artificially. If you absolutely must have part of it for research make that piece again from scratch. Consign the rest of the whole infectious replication-competent particles to the furnace where they belong.

EDIT: I found a paper in which smallpox DNA was extracted and viruses observed via EM from a 50 year old fixed tissue sample from a pathology lab that was not from one of the aforementioned collections. No word in the paper on if it was potentially infectious or just detectable levels of nucleic acids and particles. These things could be more complicated to 100% securely destroy than we thought...