Spam is a problem for everyone and is extremely difficult to control. The problem for parents is that children may open the mail and meet advertisements and links that will take the browser direct to some of the most graphic images on the net. Fortunately this area is also the easiest to control. Email requires a password. Most email programs allow the password to be stored and automaticaly open the email function when we log in.
Prevention is to turn off the automatic password function.
However if your kids have thier own email then the only way to prevent them following the links from sex advertisements, is by being there when the mail is opened. For those of us who have children who are not mature enough yet to handle grown up material the answer must be supervision. There are some programs out there that will sensor the material the viewer is permitted to see. Many of them have some side effects that can vary from the comical to the direct hair raising. One famous example being one that takes the word nipple as forbidden, then cuts out all web sites that use the word in a completely innocent context, like breast feeding or even car servicing.