Prevent VPS from being used to send out spam

My VPS is being used for sending out spam This is a simple task, but an important one! My VPS is being used to send out lots of spam. I'm not sure if it is spoofing or by server access but would like advice on the best way to stop this and prevent it in the future and these measures setting up. My initial thought is to have an SPF record set up, so, initially, I need a simple SPF record implementing (v=spf1 mx –all). I will need my hostnane configuring and an SMTP banner setting and maybe also need Reverse DNS/PTR setting up. I am open to any and all recommendations about this issue and willing to implement whatever is necessary to prevent this problem. I put a simple £20 as the suggested budget but this will obviously increase depending on further recommendations and implementations. Thanks for bidding.