if you disable the xmltv feed then eventually all the events that were added via that feed will be removed (how long that takes depends on the number of days that is in your xmltv feed)
an other option might be to disable the xmltv feed, stop tvheadend, remove the epgdb file (under the .hts settings directory) and then restart tvheadend. Because there is no epgdb no historic events are available and the epg will be repopulated with only the data contained in the EIT feed.
Hein