I would like that grafana shows only the values in MB and not a mix of MB and GB. How can I force Grafana to use a single unit only?

I would like that grafana shows only the values in MB and not a mix of MB and GB. How can I force Grafana to use a single unit only?

When you are using any unit that is built into Grafana (i.e. selectable in the Unit setting), it'll adjust automatically for large/small numbers with an appropriate metric prefix.
A workaround to have always the same unit displayed instead of the metric prefix automatically adjusted is to set it as custom unit. You can do it like that:
XYCustom unit: XYThis will also work for MB, but only if you select Custom unit: MB.