Can I use Asset Denial to make my opponent discard a region from his province.
Asset Denial and regions
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Can I use Asset Denial to make my opponent discard a region from his province.
I'm unsure if exactly what you mean by "in a province" here.
If there is a region face-up in the province that has not yet attached to the province, yes, you can discard it.
If the region has already attached to the province, it is no longer "in a province" per Asset Denial's text and cannot be discarded.
On: Asset Denial