Yep.
You cannot change the locks when a tenant still has belongings in an apartment unless you have gone through the legal process to evict them.
It does not matter that the tenant refused to pay rent for June. You have a civil recourse to pursue rent. You cannot legally lock them out.
It does not matter they were not on the lease. There is never a requirement of a signed lease to create a tenancy.
AND it is also irrelevant that this was "quiet time." I'm sure they will argue they intended to be as quiet as church mice as the gathered the last of their belongings. But locking them out is certainly not designed to facilitate quiet - as it would irritate most people quite significantly.
Illegal lock outs can actually be criminal matters in some states. (This means they could have called the police and you would have been arrested if you lived in one of these states.) But in EVERY state an illegal lockout allows the tenant to sue in civil court for damages. Had you taken them to court for June's rent, you would have won. But by locking them out? You don't want to take them to court - because they will be able to countersue.
Tomorrow, when they collect their stuff and threaten to sue you - just calmly tell them you would be happy if they did so you could countersue for June's rent because THEIR lawsuit would make it clear they still considered themselves to be tenants during June.
(FYI: This is a complete bluff. An illegal lockout trumps non payment of rent, especially when the tenant is moving everytime. And BY illegally locking them out, you gave them permission to move out without notice in June. You will lose if this goes to court.)
It might cause them to think twice about filing anything against you, though.