Update: I rewrote the text on Monday 6 February.
The EU Council and Commission have opposing opinions on whether Poland can stop ACTA.
According to ZDNet, Poland may not ratify ACTA, which could spell the end of ACTA for the entire European Union:
“Tusk’s backtracking could spell the end of ACTA for the entire European Union. If Poland or any other EU member state, or the European Parliament itself, fails to ratify the document, it becomes null and void across the union.”
ZDNet added: “The European Commission confirmed to ZDNet UK that if just one member state does not ratify ACTA, the deal will not enter into force anywhere within the EU.”
But an IP Watch article of 21 December by Monika Ermert has opposite information on the ratification process:
“A spokesperson of the EU Council told Intellectual Property Watch that while member states would ratify ACTA according to their national laws, the parts of the agreement that do not touch criminal law issues would fall under the competence of the Community – and henceforth become valid across all member states.”
The EU Commission and Council provide opposing information on the ratification process.
If we look at the Lisbon Treaty, the Union is competent with regards to civil and border measures.  This could suggest that the Council is right.
But the Council spokesperson overlooked something. The Commission is not exercising the Union’s full competence, it presented the ACTA proposal as a mixed agreement. Such agreements require the common accord of the Member States. Poland does have a veto.
The Commission proposal says:
“6. ACTA contains a number of provisions on criminal enforcement that fall within the scope of Article 83(2) TFEU. Those parts of the agreement, in distinction to those parts falling under Article 207, fall under the area of shared competences (Article 2(2) TFEU). Where a matter falls under shared competence either the European Union or Member States may legislate and adopt legally binding acts. Regarding the signature and conclusion of ACTA, the Commission has opted not to propose that the European Union exercise its potential competence in the area of criminal enforcement pursuant to Article 83(2) TFEU. The Commission considers this appropriate because it has never been the intention, as regards the negotiation of ACTA to modify the EU acquis or to harmonise EU legislation as regards criminal enforcement of intellectual property rights . For this reason, the Commission proposes that ACTA be signed and concluded both by the EU and by all the Member States.”
The last sentence says: “both by the EU and by all the Member States.”
The ACTA negotiations started under the European Community Nice Treaty. While the European Court of Justice had given the Community the right to make criminal law (yes, sounds a lot like a coup d’état), the Community member states did not accept that. The Commission left the negotiations on the criminal measures to the member states.
During the negotiations the Lisbon Treaty entered into force, making the Union in principle competent to make criminal law. But the Commission did not want to push the limits, avoided the risk of a competence fight (which earlier had killed the IPRED2 proposal).
So ACTA is presented as a mixed agreement. The rules for that can be found in the Nice Treaty. Under (Nice) Treaty establishing the European Community art 133.6 “the negotiation of such agreements shall require the common accord of the Member States”. Common accord: the EU member states do have a veto.
Poland, or any other member state, can stop ACTA.
The Parliament can stop ACTA as well.
And Commission, Council, Parliament and member states can ask the European Court of Justice whether ACTA is compatible with the Treaties.
 Article 207.4 lists some exceptions where unanimity is needed.