I don't think you can. It's her visa, and as far as I know, you can't cancel it any more than you could cancel your neightbour's visa. Why do you want to?

What type of visa?

Spouse cp

You can't cancel a visa. But if you can report it to the immigration giving details of her name and passport. The visa is only valid for the uk, therefore, travel to another county with the exception of her own is not permitted. Also if your wife decides to leave the uk for a trip home, entry back to the uk will not be granted. You would need to report to the immigration service in writing via fax stating that u no longer wish to continue in a relationship now or in the future, and upon entry back to the uk your partner would be stopped and asked questions. They would then call u and ask if the details you gave earlier are still your opinion and if so then entry would not be granted