Sentence:  The most <WSD>persistent</WSD> tormentor was Alex Oxlade-Chamberlain, who scored a hat-trick in last month's corresponding fixture in Iceland. His ability to run at defences is instantly striking, but it is his clever use of possession that has persuaded some shrewd judges that he is an even better prospect than Theo Walcott. 
Meaning of the word marked with <WSD> tags:  Obstinately refusing to give up or let go.