I think not allowing people to leave a guild would be quite a major designflaw, for various reasons
What if the guild leader turns out to be a jerk that doesn´t use the plots for their intended purpose?
What if the project changes direction mid-build and you don´t like the new idea?
What if I really want to leave a guild but the leader won´t kick me because he wants to keep my plots? Honestly I`d just start to grief the build until he´s forced to kick me…
I think a guild that has an insufficient amount of free beacon plots to give you back your invested beacons should behave like a bankrupt company. Meaning that if you leave a guild that doesn´t have enough free plots to pay you back the plots you donated to the guild it effectively has a “beacon-dept” to you. This would mean that all additionally donated plots directly go into your “plot pool” until the guild has paid off their depts to you. And only after a guild has paid all its depts it can increase its available beacon pool again.
Example:
This system could be extended by a rule that automatically closes guilds that have a too high dept %. (Just like the tax-office IRL)