Article 50 of the Treaties of the European Union (the article which governs leaving the EU) has a clause which explicitly mentions that rejoining the EU is possible after leaving it:
- If a State which has withdrawn from the Union asks to rejoin, its request shall be subject to the procedure referred to in Article 49.
Article 49, in case you wonder, is the general admission process which applies to all would-be new EU members. It requires unanimous consent from the EU council members and simple majority from the EU parliament. That means any EU member could veto the UKs re-admission, but currently I wouldn't know of any EU state having a good reason to do so.
So you can dismiss the statement "UK will never be able to rejoin the EU" as FUD.
You can of course consider the political problems of re-joining the EU after a referendum just came to the conclusion that the UK population does not want to be a part of the EU. While referendums in the UK are non-binding due to parliamental sovereignty it would theoretically be possible to act against it. But doing so might be considered political suicide. It will likely take a new referendum to rejoin the EU, and until circumstances have considerably changed there will likely be harsh resistance against another one.
On the other hand, there are also all the special privileges the UK used to have in the EU, like opting out of the Schengen agreement, opting out of the Euro zone, a rebate on financial contributions to the EU and many more. All of that would need to be renegotiated in case of a readmission.
So while it is wrong to say "The UK can not rejoin the EU", one might say that "The UK can not rejoin the EU and regain all the special privileges it had".