Standard bank is trying to rob people of their hard earned money

I had a Cheque account with standard bank, in 2019 i called them to close the account since i was not working and the branch in Cosmo City was closed so i could not go inside the bank, so now some lawyers are calling me and telling me I'm owing standard bank R7500. how can a cheque account of just R100 interest per month rise to R7500. i never had a loan or credit card with them. so now they just want to rob me of my salary. this is really crazy. im not happy with how standard bank handle people. i left standard bank because of the same thing that happened early 2019 when i was on maternity leave and i was not getting any salary and they told me I'm owing them R2500 in just 4 months of no salary in my account. and they solved it and cleared all the conflicts. that's when i called them to cancel my account after returning back to work, now they are telling me I'm owing them R7500