It has been shown a Liberated Borg cannot be reabsorbed against their will.
I'm not sure of the reasons, but their altered nature means they are immune to assimilation.
After all, Seven of Nine had to voluntarily rejoin the Collective, the Borg Queen had no means to force her hand.
Also, there are ways of rendering certain races immune to assimilation mind control.
Ironically, for such a mentally disciplined race, Vulcans are weaker to assimilation then humans.
Tuvok was easier to absorb then Janeway and Torres.
It has NOT been shown, the Borg Queen didnīt want to reassimilate Seven, because she wanted to use her individualtity against her. To show her that the Collective is better. Never was it stated anywhere that once assimilated people could never be reassimilated. Also you might wanna remember Hugh and the fact that in First Contact the Queen decided to make Picard an ordinary drone, that never happened because of data.
The only race that has been shown to be immune to Borg assimilation is the Undine.
Re-assimilation is entirely possible. The only case you can make against this would be improvements in resisting assimilation by research but that would not be something exclusive to liberated Borg. The treatment the Voyager crew got would apparantly work for everyone and I imagine Starfleet would try and improve on those treatments.
At best, an accolade that grants a resistance against Borg assimilation / powers from the STFs would be an option. It wont change the fact the Borg STILL have the ability to stick long tubes into you and can slap their cyborg implants into you.
Nevermind, them adapting their Assimilation process to counter-act any form of treatments.
Afterall, we already see one case of the Borg finally being able to assimilate Undine which would require a considerable improvement in assimilation technology to achieve than adapting to treatments on species they have studied at great length.