Prevent clients from using mods on a server

If you want a vanilla server, you cannot prevent cheats. If you use forge / bukkit, you can use NoCheat or similar plugins.

The texturepack part is a bit more technical. The way they work is that minecraft looks for textures inside. For example, if it finds grass.png(Example), minecraft will replace the original grass texture.

Lets say my texturepack is as follows:

• Grass.png
• CraftingTable.png
• Dirt.png

What happens is that minecraft looks for all textures, and only finds those 3. In the end, you end up with the new Grass, Dirt and Crafting Table. The rest of the blocks are the regular texture. So what you could do is get the "Vanilla Texturepacks" where no textures are changed. Because of the way texturepack sorting works, the server's texturepack is loaded last, overriding the client's texturepack.

Note:

1. Use resourcepacks. Texturepacks are outdated.
2. This also applies to entites.
3. The file names are examples. Do not follow them.

It is possible to use NoCheat/NoCheat+ to prevent cheating, and Orebfuscator to prevent X-raying; however, to my knowledge there is no way to completely ban all mods.

I don't think it's possible to force a certain texture pack.