First of all, have you told her to stop ringing, if you have and she still persists, contact the police, they need to be made aware of the circs, and could will speak to her and warn her (giving her a sec 2. harrasment warning), if she then breaches this, they then have grounds to arrest her.
this will depend on what circs are occuring, and if the police feel she has been causing you harrasment/alarm/distress.
Other avenues are contacting the telephone company and seeing if they can block the number, or solicitor to take civil action...
check this link, if this helps
http://www.askthe.police.uk/content/Q151.htm
either way, seek advice... hope this helps