There are several possibilities to choose which entries should be synced and which not:
org-caldav-select-tags, which is directly mapped to
org-export-select-tags, so see its doc-string on how it works.
org-caldav-exclude-tags,
which is mapped to org-icalendar-exclude tags.
org-caldav-skip-conditions. The syntax of the conditions is
described in the doc-string of org-agenda-skip-if.
org-caldav-days-in-past to the number of days you want to keep in
the past on the remote calendar. This does not affect your org files,
it works just as a filter for entries older than N days.
Note however that the normal org-agenda-skip-function(-global) will
not have any effect on the icalendar exporter (this used to be the
case, but changed with the new exporters).