TAKOBOTO

you shouldn't, one should not
美術館で写真を撮ってはいけない。/You shouldn't take picture in the museum.

Add formation 

どうぶつをいじめてはいけません。

One must not harass the animals.

ここでたばこをすうってはいけません。きんえんですから。

You are not allowed to smoke here, because this is a no-smoke area.

おさけをのんではいけません。

You are not allowed to drink alcohol.

ここにじてんしゃをとめてはいけない。

It is forbidden to park bicycles here.

「あなたはしおからいものをたべてはいけないよ」といったけど、かれはたべものにたくさんのしおをくわえた。

Although I told him "You're not allowed to eat salty!" he added a lot of salt in his food.

びじゅつかんでしゃしんをとってはいけない。

Please refrain from taking pictures in the museum.

そのこきゃくにおうたいしてこれいじょうじかんをむだにしてはいけない。

Don't waste any more time responding to that customer.

ほかのひとのたすけをあてにしてはいけない。

You shouldn't count on others for help.

ドアをあけっぱなしにしてはいけません。

Don't leave the door open.

がいけんはあまりあてにしてはいけない。

You must not rely too much on appearance.

きんきゅうのばあいいがいはわたしのえんじょをあてにしてはいけません。

Don't look to me for any help except in case of emergency.

きみはしっぱいをふうんのせいにしてはいけない。

Don't set your failure down to bad luck.

きみはものごとをちゅうとはんぱにしてはいけないよ。

You shouldn't do things by halves.

「せんせい、ここであそんでもいいですか。」「いいえ、［あそんでは］いけません。」

- Teacher, is it okay to play here? - No, you cannot .

Gerund form (Ｖて形) + は + いけない
Gerund form (Ｖて形) + は + いけません
Doing that, it cannot work. => You should not do that.

赤毛

This entry has a typo in it. Clearly the "形" is not part of the entry (unless it is completely silent. )
I suspect that the "形" in "Gerund form (Ｖて形)" was made part of the entry by mistake.

Kurt Schmucker

Technically 行けない is as ok as is いけない. But since it is part of a grammatical structure, the "directed walking" aspect of 行 is unpreferable, thus Hiragana-form is found more often.

adnim

...?
て形 means te-form, so it doesn't seem to be a typo. Unless I'm missing something here.

ochacha