It appears that in Greece, parties can expel MPs for not voting with the party:

Twenty two PASOK MPs and 21 New Democracy deputies voted against the bill. In both cases, those lawmakers were expelled from their parties. 

Former Transport Minister Makis Voridis and Deputy Mercant Marine Minister Adonis Georgiadis went against the line of their party, Popular Orthodox Rally (LAOS), by voting for the bill. Both were expelled.

It was the first time in Greek parliamentary history that so many lawmakers were ousted from their parties on the same night.

Certainly a dramatic situation, but party ownership of seats seems like a legitimate rule, given the fact that Greece uses a closed-list proportional representation system.  When voters cast a ballot, they are clearly making a choice for a party, and not an individual candidate. If Greece used Open List PR, then a case could be made that the individual was driving the party’s capture of that seat, and such a rule would seem less democratic. Some countries with majoritarian systems, such as Kenya and India, do have rules that prevent floor-crossing.  In most of these cases, however, the seat is vacated and a by-election is held. The party can not just fill the vacant seat with its next in line candidate. In Lesotho, which has both PR and single- member districts, the PR MPs are prevented from switching parties but the SMD candidates are free to do so.