Scrying mirror as a portal/gateway

I’m curious about this. When a spirit you call manifests in the mirror, will it be a possible gateway/portal for other entities to come through uninvited? If so, how to prevent this? Covering the mirror after ending ritual? Banishing? (if one does come through), what else?

I used to read experiences of others literally throwing away their scrying mirrors but I rather take advice from BALG forum.