How do i stop another guy from troubling my gf and stalking on her..she 2 is troubled but cant fight?

my gf plays tennis at school & u know how their outfit is. she looks really hot, but there's dis another guy who's been pestering her & has got her no frm somewhere & has been calling her all da time 2 ask her out..she of course is 100%commited 2 me (we've been 2gether for 5 yrs and dis is our 6th)& always refuses but is still troubled by him..this had been going on for mths but she mustered up the courage to tell me this only last mnth & asked me not 2 get in2 a fight.(she really is girly and senti and hates fights) now,i'm not da big bully but definitely stronger dan most guys & can beat his brains out if i want, but my gf told me she doesn't want fights ,so i just go with her 2 her practice 2 make sure she's safe..but next week i've 2 go 4 my football match(i'm da cptn).so how do i ensure bfore i go dat he learns 2 stay outta her way(without her knowing about my small 'talk' with him).please b quick.i dont have time