You can either remove users from individual workspaces or from your account entirely.
Removing users from a workspace
You will need to have Workspace Owner permissions to remove users from a workspace.
Go to the 'Members' section of the workspace that you wish to remove the user from. You will see a list of everyone who has access to the workspace. To remove users, simply click on the 'Remove' link.
NOTE: This will only remove the user from this workspace, not from the account.
Removing users from your account completely (including all workspaces)
You will need to be an Administrator to remove users from your account. When you remove users from your account, you will automatically be removing them from all of the workspaces that they are a member of as well. To do this, go to the 'User Security' section of your account 'Settings'.
You will see a list of all users in the 'People registered on this account' section - simply find the user in question and then click on the 'Delete user' link against their record.