Because editing permissions enable you to merge, the only way to not allow HubSpot users to not merge is to take away their editing rights.
Dependent on how you use Hubspot, as @Jnix284 stated…merging cannot be undone. So because merges are non-reversible, it should have its own level of permissions and not tied to “editing”.
We have an unspoken rule to not merge, however that doesn’t mean it doesn’t happen.