Groenewald and Lubbe Bullying me into acknowledging an Account.

Hello I'm new to this. I have had a bad experience with Groenewald and Lubbe.. Collections for FNB. They are bullying me into acknowledging a loan account.. I have sent them emails and have phoned their call centre and have tried explain to them that I am Not acknowledging the account but they keep on Spamming me with sms''s and emails saying if I don't phone them to make an arrangement for payment I will be handed over to the Sheriff or I have been lodged with the Sheriff. Are they allowed to bully people into acknowledging and forcing them to pay a debt that has been prescribed.