The Fundraising Regulator has highlighted the fact that some charities are not managing suppression lists properly, and this can lead to complaints and enforcement action.
The Code of Fundraising Practice specifies that charities must stop sending direct marketing communications to individuals after an FPS request has been received, or an individual has opted out in other ways.
The Fundraising Regulator has published a useful guide on managing FPS requests, and you can read at https://www.fundraisingregulator.org.uk/fundraising-preference-service/charity