Changes to common property require a Special Resoloution carried by a 75% majourity at a GM (EC cannot authorize changes).
You could advise them that if they keep making changes without an approved SR you will take them to CTTT to have them pay to restore the situation.
Also advise the SM not to pay any invoices or authorize any work unless a SR has been approved at a GM (for changes not general maintenance). Work can be approved retrospectively but for changes it requires a SR and so make sure this is what is on the agenda.
If these people have the numbers you have a problem.