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