Non renewable memberships
I don't think non-renewable memberships are possible at the moment. Your options are to play with CiviDiscount and/or CiviRules. 

You could offer a discount to get the first period free, or as petednz suggested look at negative discounts. I think the negative discount (fixed amount not %age) will need to be applied to existing members.

You could also have people add them selves to a group (say Trial), which gives the same privileges that the membership would provide, then perhaps CiviRules could be used to change the group to another say Trial expired, after 6 months.