English-language equivalent to the Russian idiom, 'Not let someone within firing distance near X'

Solution 1:

We have the saying "within spitting distance".

. This guy shouldn't be allowed within spitting distance of X.

within spitting distance

= close, near, beside, alongside, close by, just round the corner, within sniffing distance (informal), a hop, skip and a jump away (informal) • a restaurant within spitting distance of the Tower of London

http://www.collinsdictionary.com/dictionary/english-thesaurus/within-spitting-distance

I'm not sure it's the best option for your specific example.

Another possibility is "within a mile of".

. This guy shouldn't be allowed within a mile of children.

This doesn't literally mean a mile, just a long distance.

Solution 2:

There is a phrase I wouldn't touch it [him/her] with a ten foot pole

(verb phrase)

To be loath to have anything to do with; be suspicious or apprehensive; reject : If I were you I wouldn't touch that proposition with a ten-foot pole

[1909+; semantically akin to the proverb advising us to use a long spoon when we eat with the devil; an earlier and once more common version spoke of a forty-foot pole]

Dictionary.com

This phrase is sometimes applied to potential (or not so potential) romantic partners.

If you are specifically looking for the dangerous aspect, this doesn't supply it.

Supplement

Another phrase concerning keeping one's distance is I would avoid him [it/her] like the plague

Evade or elude at any cost, shun. For example, Since Bob was taken into police custody, his friends have been avoiding him and his family like the plague.

This seemingly modern expression dates from the Latin of the early Middle Ages, when Saint Jerome (a.d. 345-420) wrote, "Avoid, as you would the plague, a clergyman who is also a man of business." The plague, a deadly infectious disease in his day, has been largely wiped out, but the term remains current.

American Heritage Dictionary of Idioms found at idioms.thefreedictionary.com

Solution 3:

A man or woman of this variety should be kept at arms length... this is a bit closer than a 10 foot pole and a little further than spitting distance; you might also say within a stones throw, a distance very similar to spitting distance. It's all about distance and the understanding that coming closer might cause a violent action.