Can't delete bootcamp partition

Hello!

I've installed a Windows partition on my MBP a few years back, but as I don't use it anymore, I would like to remove it. Unfortunately, when I launch Boot Camp Assistant, I get the following message : the startup disk cannot be partitioned or restored to a single partition.

Is there any other way to do it? I've read a few posts on here and on other forums, but I'm afraid I'm gonna make a mistake and end up making the situation worse than it is right now...

Thanks in advance !