Many of my customer setup their email addresses to forward to their main email. However, after forwarding the email is remaining on my server.

In WHM, how do I have the system automatically purge emails after they have been forwarded to my customers

just tell customers to not create local email aliases for the emails they will be forwarding. i.e., they want "sales -> user" forward, then they just create the "sales@domain.com -> user" forward rule without creating the sales@domain.com pop box, then exim would automatically discard email after forwarding to prevent loops with the message
discarded <sales@domain.com> R=has_alias_but_no_mailbox_discarded_to_prevent_loop

Any other suggestions?

Originally posted by floyd06
However, after forwarding the email is remaining on my server.

How do you mean by "remaining on my server"? Once forwarded, the emails get collected in the main email account. Isn't that what your customers want?

There isn't a way. piramida's solution is the correct one. Customer's will work it out quick enough when their web space fills with duplicated e-mail. Just explain to them that forwards shouldn't have POP boxes set up for them.