england-and-wales
The offence of aggravated trespass, s68 Criminal Justice and Public Order Act 1994, seems to fit the circumstances on the face of them.
(1)A person commits the offence of aggravated trespass if he
trespasses on land and, in relation to any lawful activity which
persons are engaging in or are about to engage in on that or adjoining
land, does there anything which is intended by him to have the effect—
(a)of intimidating those persons or any of them so as to deter them or
any of them from engaging in that activity,
(b)of obstructing that activity, or
(c)of disrupting that activity.
It doesn't provide for a defence or lawful excuse.*
"Land" includes buildings: DPP v Chivers [2010] EWHC 1814 (Admin).
My understanding is that the people who set the alarms were invited to the banquet. However, the fact of the invitation of the banquet did not give them licence to release electronic alarms tied to balloons at the banquet.
The act need not be illegal in itself. The necessary element is that the person carried out the act with the intention of intimidating, obstructing or disrupting. The people who released the balloons seem to openly admit to intention to disrupt the banquet.
However, I doubt there will be any prosecutions in these particular circumstances. In which case the behaviour was legal.
*In contrast, Penalty for wilful obstruction s137 Highways Act 1980 starts, "If a person, without lawful authority or excuse". This is what Just Stop Oil rely on when they slow-walk along a road. They have the excuse of exercising the right to freedom of protest; they excercise their rights within the constraints so far tolerated by the authorities. Fewer people seem to be aware of the "lawful authority or excuse" than claim awareness of the Highways Act.