Council officials and police were forced to
step in last year – issuing a closure order for the flat banning visitors.
to step in : means to intervene - to act
issuing : nice verb - to issue; a public office can issue a document, a school can issue a certificate
banning : it in its -ing form, the verb is to ban to prohibit, here what's important is the structure it is equivalent to saying a flat closure order that bans visitors.
"from the news in the Sun"