after 2 hours of research im quite sure, that this isn'T possible the easy way, but perhaps anyone has a hint for me.
I like to remove all watchers of all closed issues.
Actually this is not the right answer for your case. I'm telling you what I have done for same case. from the Jira Tools (next to Export button) everyone can resolve his own watched issues. that is enough to reopen bulk issues and then using Stopwatching feature is available within the tools. remember to untick the email sending option for avoing bulk email. This only can change 1000 items so you need to repeat that.
Some people instead of answering the question just making another question. In my point of view the reason to do this is to use a dashboard gadget for watching live issues. We had many projects and I was watching huge amount of issues as Jira administrator. This was also a problem for other users. So I announce all of them for a while to go and stopwatch any cases they do not want. so I made a quick guide for using Jira Tools. I managed my problem in this way.
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