How to prevent somebody from leaving a group?

Hello, I have a question...

In my forum I have created a couple of groups which all have a specific colour and a specific rank which are given to members who have a specific role...

For exemple:

green = the guitar maker group
blue = the moderator group
orange = hobby guitar builders
yellow = contributors
red = administrators group

Now recently some members of the orange group delated themselfes from the group and took of their rank and I was wondering if there is a way to prevent them from having the ability to do so?

Thank for your help!

Sorry there isn't.