Since IPv6 Connection Feature (IPoE) is offered as an accessory feature of Asahi Net's connection services, you cannot deactivate just the IPv6 Connection (IPoE) while subscribing to an applicable connection service. 
You can deactivate the faeature if you are canceling the applicable connection service due to switching of ISPs, etc.

For your information, you can activate/deactivate IPv4 over IPv6 Connection (DS-Lite) (which is an accessory feature of the IPv6 Connection Feature), according to your use. 

You can make various requests for IPv6 Connection Feature on your MyPage. 
IPv6 Connection Feature Check/Request