This ICAO document provides a good overview of both the Tokyo Convention and the Montreal Protocol and its purposes:
https://www.icao.int/MID/Documents/R...c%2010117).pdf
In para 2.4.2. the definition of 'in flight' for the purposes of both Tokyo and Montreal is provided. Note that neither convention/protocol excludes any criminal jurisdication exercised in accordance with national law. I think that you can translate that as: any offences that take place on the ground in a third country
can be dealt with by the national laws of that state, but Tokyo/Montreal allows the state of registration to assume jurisdiction over the situation and prosecute accordingly. Montreal adds the option for the state of landing to do this.
Under these conventions, the Captain has some additional authority to deal with unruly behaviour and such, but this does not (fully) extend to the situation as described above, that of a (I presume) ground handler or similar entering the aircraft while on the ground in another state than the home state.