In Triib, we don't delete membership plans (just in case you ever want them back!), but rather deactivate them. To do this:
Click Members > Membership Plans > click on a membership plan > uncheck the Active box at the top of the Main Settings section.
Your disabled membership plan will not appear online or under Active Plans. It can be found under the Disabled Plans tab on your Membership Plans admin page.
NOTE: If you have active members on a plan when disabling it, it does not take them off the plan. It will keep them on the plan until they cancel.