One of the ten categories of yuchsin is Nesinim. They were the Givonim (one of the 7 nations) who converted. While they are forbidden to enter the kahal, is it an issur dioraissa or an issur dirabanan?
Our girsa in the Mishna, Makkos 13a says there is malkos for marrying a mamzeres and nesina. Rashi (Makkos, and Sanhedrin 51a) says the lav for marrying a nasina is "lo tischaten bam." This implies that nesina is assur dioraissa. Ramban says this in Makkos and it is Ritva's first explanation.
Yevamos 88 says that Yehoshua and David Hamelech made gezeiros against the nesinim. If they were already asur because of lo tischaten, why were gezeiros needed? See a few paragraphs below.
Rambam (Hil. Isurei Biah 12: 22-24) says that if one of the 7 nations converted they are mutar lavo bekahal immediately (unlike mitzrim who must wait 3 doros), and Nesinim converted but Yehoshua decreed against them that they were asur lavo bekahal during the time of the Beis Hamikdash, and David decreed they were asur even after the Beis Hamikdash because they were cruel people. Thus, Rambam holds they are only asur midirabanan, and his text in the mishna in Makkos either does not have the word nasin in it at all, as the Ritva in Makkos says in his second explanation, or the Mishna says it "agav grara," just mentioned by the way once mamzer was mentioned, as Maggid Mishna (Issurei Biah 12:22) says (and Ramban in Makkos rejects).
To the opinions that Nesinim are asur midioraissa, why were gzeiros needed? To make them like avadim so they'd be asur lavo bekahal midirabanan.
From Ramban Yevamos 88b: A further question is that the mishna in Kesuvos (29a) says one does pay knas (50 silver pieces) for raping a naarah besulah, if he rapes a nesinah, but one does not pay knas for one who converted or was freed above the age of three years and one day. If they are like shifchos, why do they have knas? Ramban Yevamos brings Rabbeinu Tam that they were like avadim, but if one is raped they get knas because they are not avadim whose bodies are owned by a master.
Rabbeinu Avraham ben David reject this for certain reasons and explains that the Nesinim who originally converted did not have a good conversion. So Yehushua and David decreed that we may not accept converts even from later generations.
Ramban's third explanation is that only the first generation of givonim were asur (because of lo tischaten) and Moshe decreed on the first generation they should be like avadim so the Bnei Yisrael would look down on them for being avadim and not mingle (and marry) their descendants, and Yehushua and David decreed they are asurim forever.
Some other things are discussed on this topic, if you're interested see Ramban Yevamos 88b and Shita Mekubetzes Kesuvos 29a inside.