As Counterpoint has mentioned, there are exemptions and you should apply these wherever possible. The legislation is difficult to read, but the Information Commissioner's Office might be able to provide specific advice. If your Clerk's salary is included as an individual item in your published accounts, you do not need to respond to an FOI request, as the data is already in the public domain. Similarly past decisions are recorded in the minutes. The Clerk's sickness record should be covered by the Clerk's right to privacy, as should many other aspects of her employment. Check GDPR for her right to object to the disclosure of this information.
It is interesting to note that the ICO defines a senior post as somebody who is making high-level decisions and spending large sums of public money, neither of which applies to a Clerk, so the enhanced disclosure rules for senior employees should not apply.
If you provide the ICO with a list of all requests received from this individual and the timescale, they should also be able to advise on whether they constitute vexatious communication.