Hi Tuan
In our association, we want to use Life plan to give certain users such as ex-President a life membership with no expiration date.
We also give out Complimentary membership (say free for 12 months) to users who have paid and attended our conference etc...
We do not want users from the frontend to be able to subscribe to both Life and Complimentary plans. When users with Complimentary plan expired, they should not be able to renew on the same plan. The users will be forced to either renew or upgrade to any other plan except for Life and Complimentary.
Does this make sense?