If I understand correctly, the following solution would be one way of going about:
string <- c("onetwo", "two", "three", "fourfive", "five", "six", "sixseven")
find.list <- list("two", "five", "seven")
# in REGEX, | also represents "OR"
find.string <- paste(unlist(find.list), collapse = "|")
gsub(find.string, replacement = "", x = string)
[1] "one" "" "three" "four" "" "six" "six"