As with many things in D&D, this answer comes in two flavors.
In terms of rules-as-written, some of the non-instantaneous, non-concentration spells do indeed point out that they can be dismissed. I believe the NDA is still in place, but in your packet you can see that these spells include Disguise Self, Light, Mage Armor, Minor Illusion, and Thaumaturgy. There are other non-instantaneous and non-concentration spells that have no explicit dismissal mentioned, such as Pass without Trace, Speak with Animals, or, as you mentioned, Spike Growth. I was also unable to find any reference in the magic rules to dismissing spells, outside of individual spell descriptions, so I would say that, by RAW, unless otherwise stated, non-concentration spells cannot be dismissed early.
This is where it gets trickier, and I've either found or imagined bits of evidence in favor of both sides. It seems that most of the spells with explicit mentions of dismissal are first-level or cantrips, implying that, if the writers of the playtest started from the lower levels, they might have forgotten that they needed to explicitly state that the spells could be dismissed. However, there are also spells such as Passwall, where it's actually explicitly stated that the spell lasts for the duration. This seems to be an intentional choice to limit the spell's utility. If you can't end it early, you can't use it to open a passage in front of you and close it behind you to escape an enemy. That the writers felt obligated to point out that the spell lasts for the duration seems, to me, to imply that other non-concentration spells without this clause can be dismissed at will. There doesn't seem to be a clear rules-as-intended ruling on this issue, at least not from the packet exclusively.
What should the ruling be?
Ultimately, it's up to your group, or the DM of your group, depending on your social dynamics. Due to the unclear intent, I might personally adjudicate that there is some kind of cost to dismiss the spell. Perhaps it takes an action, as Disguise Self does, to be dismissed. Perhaps to end the spell early, you need to cast it again, in sort of a counterspelling situation. Maybe you need to roll a magic ability check to determine if the spell can be dispelled that round.
Hopefully, we can see this issue be cleared up when fifth edition is officially released. I'm sorry that I couldn't provide a definite answer, but I hope that this gives you something to work with in terms of creating a solution for your group.