馬鹿に付ける薬はない	450	6.1%
馬鹿につける薬はない	2900	39.1%
バカにつける薬はない	1852	25.0%
馬鹿につける薬は無い	823	11.1%
バカにつける薬は無い	428	5.8%
バカに付ける薬はない	378	5.1%

- having an extra gloss is OK but the case for throwing away the others is weak.
- I think the order and sK tagging needed tweaking.

@@ -5,3 +4,0 @@
-<keb>馬鹿に付ける薬はない</keb>
-</k_ele>
-<k_ele>
@@ -9 +5,0 @@
-<ke_inf>&sK;</ke_inf>
@@ -13 +8,0 @@
-<ke_inf>&sK;</ke_inf>
@@ -17 +12,3 @@
-<ke_inf>&sK;</ke_inf>
+</k_ele>
+<k_ele>
+<keb>馬鹿に付ける薬はない</keb>
@@ -41,0 +39 @@
+<gloss>a born fool is never cured</gloss>
@@ -42,0 +41 @@
+<gloss g_type="lit">there is no medicine for fools</gloss>

Just an example in the wild that made it to the news.
Zhang Ziyi Says There’s “No Cure For Stupidity”; Condemns Outbreak Of Novel Coronavirus
https://www.todayonline.com/8days/sceneandheard/entertainment/zhang-ziyi-says-theres-no-cure-stupidity-condemns-outbreak-wuhan

Perhaps not surprising given the derogatory nature, but the more colloquial:
"there is no cure for stupid" (*sometimes* ended with "people", but usually not)
is even more popular.  Also high up: "there ain't no cure for stupid"

Not sure either of the old glosses are necessary.  I knew this common English phrase of the top of my head and went looking to confirm it, only to find it's already a tatoeba sentence as well:

https://tatoeba.org/en/sentences/show/2495367

There's an xref pointing here that will also need to be updated when concluded.

Google results for "a born fool is never cured" are almost all dictionary references, or applications of this gloss to Japanese.  Not a particularly common English expression it would seem.  Can't see why it should be worth preserving in spite of its longevity in current references.

I guess a [lit] entry is fine, but seems redundant given how close this is to the incredibly-common English gloss. (If this were [id] and not [proverb] I'm pretty sure we wouldn't bother).

@@ -42,2 +42 @@
-<gloss>a born fool is never cured</gloss>
-<gloss g_type="lit">there is no medicine for fools</gloss>
+<gloss>there is no cure for stupidity</gloss>

〈馬鹿／ばか／バカ〉に〈付／つ〉ける〈薬／くすり／クスリ〉は〈無／な〉い

Google N-gram Corpus Counts
╭─ーーーーーーーーーーーー─┬───────┬───────╮
│ 馬鹿につける薬はない　　 │ 2,900 │ 38.0% │ - sK
│ バカにつける薬はない　　 │ 1,852 │ 24.3% │ - sK
│ 馬鹿につける薬は無い　　 │   823 │ 10.8% │ - add, sK
│ 馬鹿に付ける薬はない　　 │   450 │  5.9% │ - add (daijr/s)
│ バカにつける薬は無い　　 │   428 │  5.6% │ - add, sK
│ バカに付ける薬はない　　 │   378 │  4.9% │ - add, sK
│ バカに付ける薬は無い　　 │   287 │  3.8% │ - sK
│ 馬鹿に付ける薬は無い　　 │   257 │  3.4% │ - sK (koj)
│ ばかにつける薬はない　　 │    94 │  1.2% │
│ 馬鹿につけるクスリはない │    35 │  0.5% │
│ バカにつけるクスリはない │    96 │  1.3% │
│ ばかにつけるくすりはない │    37 │  0.5% │
│ バカにつけるくすりはない │     0 │  0.0% │ - drop
╰─ーーーーーーーーーーーー─┴───────┴───────╯

@@ -4,0 +5,3 @@
+<keb>馬鹿に付ける薬はない</keb>
+</k_ele>
+<k_ele>
@@ -5,0 +9 @@
+<ke_inf>&sK;</ke_inf>
@@ -8,0 +13,13 @@
+<ke_inf>&sK;</ke_inf>
+</k_ele>
+<k_ele>
+<keb>馬鹿につける薬は無い</keb>
+<ke_inf>&sK;</ke_inf>
+</k_ele>
+<k_ele>
+<keb>バカにつける薬は無い</keb>
+<ke_inf>&sK;</ke_inf>
+</k_ele>
+<k_ele>
+<keb>バカに付ける薬はない</keb>
+<ke_inf>&sK;</ke_inf>
@@ -11,0 +29 @@
+<ke_inf>&sK;</ke_inf>
@@ -14,0 +33 @@
+<ke_inf>&sK;</ke_inf>
@@ -18,7 +36,0 @@
-<re_restr>馬鹿につける薬はない</re_restr>
-<re_restr>馬鹿に付ける薬は無い</re_restr>
-</r_ele>
-<r_ele>
-<reb>バカにつけるくすりはない</reb>
-<re_restr>バカにつける薬はない</re_restr>
-<re_restr>バカに付ける薬は無い</re_restr>

Add POS to proverbs
-*- via bulkupd.py -*-

@@ -27,0 +28 @@
+<pos>&adj-i;</pos>