We all make mistakes and it is easy to stop the wrong person from accessing your board in Ideaflip.
If you have shared a link you can reset the link so that it won't work. Do this by either clicking on the little plus symbol '+' next to the avatars at the top-right of the screen or go to the main menu by clicking the Ideaflip logo top-left of your screen and choose invite.
In the Invite panel click the links tab and at the bottom, you will see reset links. Click this to invalidate the links you have sent out. You will now need to re-invite people again with a new link.
Once somebody has joined a board you can edit their permissions. If you want to block them from the board select their role to None. Do this by opening the main menu by clicking the Ideaflip logo top-left of your screen and choose Permissions. In the permissions screen, you will see a list of all the usernames and next to them a drop-down roles option. Choose None to effectively remove them from the Board.
If you are still concerned then you can always contact support via email or the chat-bot and we can remove people manually.