In the bottom left of your Sendspark profile, you should see an option for Manage Team:
On the modal that pops up, click into the second tab that says Manage Team
To remove a team member, click the three dots next to the teammate you want to remove, and then select the remove option.
If you don't see the three dots next to the team member, this might be because they were the original owner who created the account, and you don't have the ability to remove them.
In this case, you can reach out to the Sendspark team for additional support.