If you already have a valid visa, it remains valid. The restriction is for new visas only. Existing visas are not automatically invalidated.
Further, you cannot request the embassy to cancel your visa. It can only be canceled by the sponsor (at least they have to start the process, and eventually it is marked as revoked by the Ministry of Interior of Kuwait).
If you have a permanent resident (18) or other such visa type of Kuwait, after 6 months of absence the visa is automatically cancelled (it states so on the visa page that is affixed to your passport).
If you have a paper visa (a new visa), then it will be valid for 3 months from the date of issuance. After that date (which is written on the visa paper near the bottom), the visa is not longer valid.