I've found a hackingish way of doing that. I'm assuming you've installed rabbitvcs-nautilus. I've tested it in Ubuntu 16.04.

After installing it, if you restart nautilus, you'll find entry for three types of VCS, namely git, mercurial (hg) and SVN. Here is a screenshot of context menu after default installation.

Now, to remove git and mercurial menus, Open /usr/lib/python2.7/dist-packages/rabbitvcs/util/contextmenu.py with your favorite text editor. I'm using sublime text in the example

subl /usr/lib/python2.7/dist-packages/rabbitvcs/util/contextmenu.py

Now, go to line 1329. Comment-out lines from 1329 to 1372. Which are basically menu item entries for git and mercurial. After commenting out, the lines become

File: /usr/lib/python2.7/dist-packages/rabbitvcs/util/contextmenu.py

1329:             # (MenuRabbitVCSGit, [

1330:             #     (MenuClone, None),

1331:             #     (MenuInitializeRepository, None),

1332:             #     (MenuSeparator, None),

1333:             #     (MenuDiffMenu, [

1334:             #         (MenuDiff, None),

1335:             #         (MenuDiffPrevRev, None),

1336:             #         (MenuDiffMultiple, None),

1337:             #         (MenuCompareTool, None),

1338:             #         (MenuCompareToolPrevRev, None),

1339:             #         (MenuCompareToolMultiple, None),

1340:             #         (MenuShowChanges, None),

1341:             #     ]),

1342:             #     (MenuShowLog, None),

1343:             #     (MenuStage, None),

1344:             #     (MenuUnstage, None),

1345:             #     (MenuAddToIgnoreList, ignore_items),

1346:             #     (MenuSeparator, None),

1347:             #     (MenuRename, None),

1348:             #     (MenuDelete, None),

1349:             #     (MenuRevert, None),

1350:             #     (MenuClean, None),

1351:             #     (MenuReset, None),

1352:             #     (MenuCheckout, None),

1353:             #     (MenuSeparator, None),

1354:             #     (MenuBranches, None),

1355:             #     (MenuTags, None),

1356:             #     (MenuRemotes, None),

1357:             #     (MenuSeparator, None),

1358:             #     (MenuGitExport, None),

1359:             #     (MenuMerge, None),

1360:             #     (MenuSeparator, None),

1361:             #     (MenuAnnotate, None),

1362:             #     (MenuSeparator, None),

1363:             #     (MenuCreatePatch, None),

1364:             #     (MenuApplyPatch, None),

1365:             #     (MenuSeparator, None),

1366:             #     (MenuSettings, None),

1367:             #     (MenuAbout, None)

1368:             # ]),

1369:             # (MenuRabbitVCSMercurial, [

1370:             #     (MenuSettings, None),

1371:             #     (MenuAbout, None)

1372:             # ])

1373:         ]

1374: 

I've printed line numbers here.

Here is two screenshot which shows the context menu before and after of disabling this.

Screenshot: After disabling