[ubuntu] GnuCash - How to mark vendor invoices as paid

I am using GnuCash 2.6.1 on Ubuntu 14.04.4 for a small business.

Sometimes, when I pay vendor invoices, the invoice is not marked as paid. Thus, while the check is written and the vendor report shows a zero balance, I still get a "due bills" reminder when I start GnuCash, saying that "the following 2 bills are due."

How do I manually mark a bill as paid? And what should I be doing when paying invoices so that this doesn't happen?

Many thanks,
treacl