If you compiled the mib in MIB Tools and then mapped the mib to Spectrum, you can remove the mib from MIB Tools but you cannot remove the attributes from the Spectrum database after they have been mapped.
To remove the mib from MIB Tools, find the mib in the Navigation panel and then click on the "Delete selected selected MIBs fromthe navigation panel" icon. The one with the minus "-" character. Note that this may not be available in earlier releases of Spectrum. Not sure which release this was included in.
If the attributes have been mapped, you cannot delete them from the Spectrum database. The only option is to load a catalog from before you mapped them to the Spectrum database.
There is no single place to see all the attributes you added that I am aware of.