Is there any way to only search for ongoing manga?

Don't think so. Would have to be manual changing the url https://www.mangaupdates.com/series.html?genre=Adult_Yuri&filter=some_releases&perpage=50 to some other filter.

There isn't such a filter option on https://www.mangaupdates.com/series.html?act=genresearch . Could perhaps do two searches (everything and only complete), overlap them in your spreadsheet or database program of choice and find the ones that aren't in one but in the other.

Quote from Sosseres

Don't think so. Would have to be manual changing the url https://www.mangaupdates.com/series.html?genre=Adult_Yuri&filter=some_releases&perpage=50 to some other filter.

There isn't such a filter option on https://www.mangaupdates.com/series.html?act=genresearch . Could perhaps do two searches ...

Thank you, I will do that.
It would be easier if there was also an "hide completed" option available.