hmm if they rebuy they should be forced to drop the awp as soon as they get it. Not sure but I think if you run the weapon rescriction plugin (even if you don't restrict anything) it should block the rebuy command. anywhoo if you want you can down load this version..
I added a cvar: checkrebuy
if set to 1 then it "should" block all rebuy commands.. it's quick and dirty but if thats what you need then here you go.
The attached plugin has awplimit set to 1 and checkrebuy set to 1 (both enabled) by default
let me know if it works