Keep writing after examiner told me to stop writing

So for my gcse english exam, i was double checking my punctuations after the exams had finished and notice a error so i picked up my pen to quickly correct it. One of the examiners who had not been in the room for the entire exam walks in and tells me to stop writing. All i had was a full stop left so when the examiner turned away , I added the full stop. The examiner saw it and snatched my paper away forcefully and shouted at me to stop writing.

He held me back after the exam and told me that I was accused of malpractice and that he would report to the exam board. He said that there's a high chance that my paper would be ungraded. To give you guys a background he is a first time examiner and a power hungry d*ck. I explained myself and my actions to him but all he said was " I don't care" and " This is malpractice"

Will I have my marks deducted or get an ungraded paper?

Have a look at JCQ's Suspected Malpractice Policies and Procedures, particularly "Appendix 6 Indicative sanctions against candidates".
One example they cite is, "minor non-compliance: e.g. sitting in a non-designated seat; continuing to write for a short period after being told to stop". The indicative sanction listed for that infraction is a warning.
That means "The candidate is issued with a warning that if he/she commits malpractice within a set period of time, further specified sanctions may be applied." A warning does not result in a loss of marks.

Does this apply to GCSEs though