The use of any or all weather modification machines or devices was outlawed in 1977 by the UN Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques.  This also includes your Seismo-manipulators, the Tsunamitron, the Ozonator, and even the still-under-development El Nino-mat 3000.  I'm looking at you, ENSO Man.

Some further details-- technically the wording is framed in terms of the signatory states, but Article IV specifically enjoins the signatory parties to stop any such activity within their jurisdictions.  So the fact that you're an individual and not a signatory party is not a valid defense.

Article VII also extends this convention into perpetuity, so using a time machine to circumvent the restrictions will not be effective.

HOWEVER-- the convention only refers to techniques with "widespread," "long-lasting," or "severe" effects.  A sufficiently motivated >mad scientist should be able to circumvent the first two criteria with only a little effort, and creating short-lived localized disasters is generally more effective at terrorizing a population without destroying its revenue/ransom-generation capacity.  "Severe" ("involving serious or significant disruption or harm to human life, natural and economic resources or other assets") is a little trickier, but a good lawyer might be able to argue that a specific bank that you've just destroyed and looted via a volcano generator is not really an "economic resource" per se.