There are suspicions about Poland and I believe the Czech republic refused to have one. The main question appears to be over Romania. That is the only EU or EU Accession country to sign a bi-lateral agreement with the USA agreeing not to send US troops to the International Criminal Court. The one "black prison" that is believed to still be in existence is in Romania.

Suspending a member's voting rights looks complex from what I have seen. It requires unanimous agreement among the other Heads of Government in the Council of Ministers and a two thirds majority of the Parliament.  That is to hold a meeting to consider the suspension. Once that hurdle is passed, the meeting of the CoM can pass a resolution by qualified majority vote.

As Romania is not yet a member, the procedure may be different. The Copenhagen Criteria for new members require:

    "Membership requires that the candidate country has achieved stability of institutions guaranteeing democracy, the rule of law, human rights and respect for and, protection of minorities, the existence of a functioning market economy as well as the capacity to cope with competitive pressure and market forces within the Union. Membership presupposes the candidate's ability to take on the obligations of membership including adherence to the aims of political, economic and monetary union."

There are already questions about whether Romania will be able to fully address the corruption in many of its institutions in time for the planned accession. I would think the bilateral agreement with the US not to cooperate in helping to send accused US war criminals to the ICC would have to be considered in the context of whether it guarantees the rule of law. If they had knowledge of a "black prison", I do not see how they could be said to meet the criteria. The USA might have set one up on say an air base without their consent but immediate steps would have to be taken to close it and release anyone illegally detained.

I believe that before Accession is finalised, the Council of Ministers has to unanimously agree a candidate has met the Criteria. The pressure required to get just one to object is obviously a lot less than to try to persuade 24 and the Parliament.