OK - I did a quick bit of research and found this:
EU rules stopped Britain deporting murderers, rapists and violent criminals
Firstly, it is released by the Leave campaign, which doesn't automatically render it invalid or inaccurate, but one has to question their take on it. It seems these people are using the freedom of movement to stop their deportation to their EU country of origin. This is not the EU interfering and saying the UK cannot deport criminals; If you look at the reasoning, it is saying once they are released (early), presumably because they are no longer perceived by the parole board to be a threat to society or are sufficiently reformed (a questionable assumption given the Worboys case), then they are like any other EU citizen and free to move anywhere around the EU. As I mentioned earlier, it isn't tht the EU can interfere with a deportation per se; but they have relied on other facets of EU law to make a forced deportation of a) what have been considered to be low-risk criminals due to being paroled or b) criminals who have served their time to to EU citizens only void.
Let's put it another way.. Say I deported someone back to Poland after they spent their sentence (not their record); They can lawfully head back on the bus and return anyway. This applies to all EU members states.
Although, "Theresa May, the Home Secretary, admitted the Government needs to do more but said a record number of foreign offenders were deported last year." The EU certainly won't intervene in deportation of non-EU nationals, and their intervention in deportation of EU nationals is not based on an inherent jurisdiction over deportation law, but the fact that it was infringing the right of freedom of movement. There is nothing to prevent the UK deporting to the home country while they are incarcerated - the EU home country is, as I understand, under a duty to keep them incarcerated for the period of time they were to be subject to that country's parole regime.
Note, the article also said the EU were willing to give a bit on the deportation after serving jail - this would apply to all member states and would require either a) a relaxation of the interpretation of EU law; or b) rewrite of EU law... I will admit, if the EU were more flexible in their approach to things, then maybe the vote to leave the EU may not have succeeded...