Merged trees in error

If the merge was of two profiles that were not really the same person, then report here https://www.geni.com/discussions/245980 with a link to the Profile itself or the View Merge screen from its Revision Tab.

If the merge was of two versions of the same person, then it stays and you correct the errors.
Not sure if "we now have many incorrect relatives" means a date of birth or death is wrong or etc. in which case you just edit the profile, preferably providing documentation for the correct info, or means ??

Not sure what has gone on but somebody has managed to add a lot of extra children to my Great Grandfather and I just want to remove them

First, stop just deleting things.
Use the revision history to understand how a merge happened and what to ask to be unmerged.

When I look at the revision history this merge seems potentially in error:
https://www.geni.com/merge/view?revision_id=78091237320