It's that simple -- either they get a lone-man project, or they have to go. Get them away from your team. They are not only not doing their part, they erode team morale and practices.
Now, if it seems you have to fire 50% of your team, then...
Why some refuse? Do they have no time? Are they burnt out? Are reviews about something they have no experience with? Do they think it's a waste of time, if so why?
Agile methodology will help here -- I'm assuming your constantly work against silos (i.e. to reduce bus factor), and individuals in your team are involved in what others do.
Work to ensure individual merge requests are quite small. If it's more than 1 screen of changes, it needs a stand-up or lightning talk to explain what is being done. If it's 10 pages, it needs a presentation with slides and architecture diagrams.
Does everyone in question work on the same project?
Is the project already buried under mountain of technical debt?
Do they believe in the project and continuous improvement?