They had thrown in their lot with the heathen Gentiles, let them then become Gentiles, let them be branded with their mark, let them, by being made bald, be stamped as those who are no longer citizens of Jerusalem, but who have become outcasts and foreigners.
Then, when this was done, Nehemiah calls them to him, and makes them take a solemn oath before God, that from that time forth they will never fall into the same sin again: 'I made them swear by God, saying, Ye shall not give your daughters unto their sons, nor take their daughters unto your sons, or for yourselves.' Then he reminds them how dreadful the consequences of the same sin had been to no less a person than their great and glorious King Solomon, the wisest of men, the beloved of his God.

Even Solomon had been drawn aside into sin by his love of heathen foreigners, or outlandish women, as Nehemiah calls them, women living outside his own land.

If he fell, if he the wisest of men, if he the beloved of his God, was led astray, was it likely that they could walk into the very same trap, and escape being caught and ensnared by it?
'Did not Solomon King of Israel sin by these things?
Yet among many nations was there no king like him, who was beloved of his God, and God made him king over all Israel: nevertheless _even him_ did outlandish women cause to sin.

Shall we then hearken unto you to do all this great evil, to transgress against our God in marrying strange wives ?' Did Nehemiah then break up the marriages which had already taken place, and send the wives away?
We are not told that he did.

Probably he only insisted, and insisted very strongly, that no more such marriages should take place.