Document library permissions

[Alice_M85]

I have a user within our organisation that has read only permissions within sharepoint. He has the library synced to his file explorer and can edit documents within the files. I checked if he could within the sharepoint site on the web and he could edit documents and they would save if auto save was on.

I have checked all the permissions he has and there is nothing that gives him access to edit any documents (see attached)

Does anyone know why it would be doing this and how I can stop it?

[#2]

I see this is not the read-only permission but rather custom permission is created with is name Web-only limited maybe there is an option to check that the user has edit/modify access so it is not just this user but anyone in the SharePoint Security group will have that access.

you can name a SharePoint Security group like Access blocked but in the option, you can select full control while the display name says access blocked but that is just a name the important is the options ticked.