If you have owner access: Business Profile settings → People and access → select them → Remove. Two catches: only owners can remove owners, and a PRIMARY owner can't be removed until primary ownership transfers. Add/confirm yourself as owner, transfer primary to your business account, then remove the stragglers.
If the ex-agency or ex-employee holds primary ownership and won't cooperate: request access through the profile's 'Own this business?' flow; Google emails the current owner, and if they don't respond within about a week you can often claim it through the follow-up process with proof you control the business.
Then apply the standing rule so this never recurs: primary owner = a business-owned account; every helper, forever, gets manager access only.