There's no way to set a strict limit, but you can set a billing alert to $1 for example. This will notify you as soon as you have used all your included minutes in the plan. You can then disable access through your account page.
 
You may set a billing alert notification from your account page after logging in at https://appetize.io/account
 
The minutes that come with a plan are meant to be "starter packs".
 
More info here:
 
Let us know if you have any questions!