If you didn't invite Sembly manually, you probably connected your calendar to Sembly upon registration. You can disconnect your calendar in settings to stop Sembly from joining. Follow this link to access calendar settings. Click three dots next to the calendar you have connected, in a dropdown select Disconnect. Your meetings will disappear from your meetings list during 2-3 minutes.
If after a while you still have future meetings on your list, check if you invited Sembly to it using another method. Go to your calendar and check the attendee's list.
If you have other members in your workspace, Sembly may have been invited to this meeting by someone else.
You can also toggle off the Attend in the meeting slot to cancel the invite.