Sarah Rumbelow, who suffers from ADHD and Asperger syndrome, was sentenced to two and a half years in jail, reports the Daily Mail.
The court heard how in 2011 the 21-year-old sent emails, tweets, SMS messages and letters to Marcel that threatened her with physical and sexual violence.
Earlier this year, Rumbelow was charged with harassing the 37-year-old and issued with a restraining order. Her sentence was subsequently adjourned so it could be assessed as to whether her mental health issues could be treated and a non-custodial sentence issued.
However, upon being granted bail and released from custody, Rumbelow immediately began sending more abusive messages to Marcel, including one to her sister saying: "Your sister is finished. Rosie will get hers soon. Watch this space."
The actress, who plays surgeon Jac Naylor in the BBC One show, was in court for the sentencing and told reporters: "It was a just sentence. Now I just want to get on with my life."